use of undeclared identifier 'i'

[stavinsky]

there are 2 errors during build. removing this variable makes the code build successfully.

/Users/stavinsky/dev/build/libsigmf/external/flatbuffers/src/idl_gen_rust.cpp:499:12: error: variable 'i' set but not used [-Werror,-Wunused-but-set-variable]
/Users/stavinsky/dev/build/libsigmf/external/flatbuffers/src/idl_gen_rust.cpp:509:9: error: use of undeclared identifier 'i'