src/thread/context.hpp: fix to build with boost 1.62

[piem]

This change was all it took to build fc against libboost-all-dev 1.62.0.1 (debian stable).

See also bitshares#5

[piem]

212a749 in include/fc/rpc/state.hpp avoids this error when building with g++ 6:

[ 10%] Building CXX object libraries/fc/CMakeFiles/fc.dir/src/rpc/state.cpp.o
cd «builddir»/libraries/fc && /usr/lib/ccache/g++   -DHAS_BZIP2 -DHAS_ZLIB -I«builddir»/libraries/fc/include -I«builddir»/libraries/fc/vendor/diff-match-patch-cpp-stl -I«builddir»/libraries/fc/vendor/websocketpp -I«builddir»/libraries/fc -I«builddir»/libraries/fc/vendor/boost_1.51/include -I«builddir»/libraries/fc/vendor/cyoencode-1.0.2/src -I«builddir»/libraries/fc/vendor/secp256k1-zkp -I«builddir»/libraries/fc/vendor/equihash -I«builddir»/libraries/fc/vendor/equihash/include -isystem «builddir»/libraries/fc/vendor/secp256k1-zkp/include  -DIS_LOW_MEM -DCHAINBASE_CHECK_LOCKING -DCLEAR_VOTES -std=c++11 -Wall -fno-builtin-memcmp  -Wall -fnon-call-exceptions -DWEBSOCKETPP_STRICT_MASKING -DBOOST_ASIO_HAS_STD_CHRONO -DLOG_LONG_API -DLOG_LONG_API_MAX_MS=1000 -DLOG_LONG_API_WARN_MS=750 -O3 -DNDEBUG   -Wall -fnon-call-exceptions -o CMakeFiles/fc.dir/src/rpc/state.cpp.o -c «builddir»/libraries/fc/src/rpc/state.cpp
«builddir»/libraries/fc/src/rpc/state.cpp: In member function ‘fc::rpc::request fc::rpc::state::start_remote_call(const string&, fc::variants)’:
«builddir»/libraries/fc/src/rpc/state.cpp:47:69: error: no matching function for call to ‘fc::rpc::request::request(<brace-enclosed initializer list>)’
    request request{ "2.0", _next_id++, method_name, std::move(args) };
                                                                     ^
In file included from «builddir»/libraries/fc/src/rpc/state.cpp:1:0:
«builddir»/libraries/fc/include/fc/rpc/state.hpp:7:11: note: candidate: fc::rpc::request::request()
    struct request
           ^~~~~~~
«builddir»/libraries/fc/include/fc/rpc/state.hpp:7:11: note:   candidate expects 0 arguments, 4 provided
«builddir»/libraries/fc/include/fc/rpc/state.hpp:7:11: note: candidate: fc::rpc::request::request(const fc::rpc::request&)
«builddir»/libraries/fc/include/fc/rpc/state.hpp:7:11: note:   candidate expects 1 argument, 4 provided
«builddir»/libraries/fc/include/fc/rpc/state.hpp:7:11: note: candidate: fc::rpc::request::request(fc::rpc::request&&)
«builddir»/libraries/fc/include/fc/rpc/state.hpp:7:11: note:   candidate expects 1 argument, 4 provided
libraries/fc/CMakeFiles/fc.dir/build.make:926: recipe for target 'libraries/fc/CMakeFiles/fc.dir/src/rpc/state.cpp.o' failed
make[2]: *** [libraries/fc/CMakeFiles/fc.dir/src/rpc/state.cpp.o] Error 1
make[2]: Leaving directory '«builddir»'
CMakeFiles/Makefile2:199: recipe for target 'libraries/fc/CMakeFiles/fc.dir/all' failed
make[1]: *** [libraries/fc/CMakeFiles/fc.dir/all] Error 2
make[1]: Leaving directory '«builddir»'
Makefile:116: recipe for target 'all' failed
make: *** [all] Error 2

[abitmore]

I'd rather change request request{ "2.0", _next_id++, method_name, std::move(args) }; to use a simple multi-field constructor.