🐛 Fix dub test missing strcmp

[JesseKPhillips]

$ dub test

luad/stack.d(588,33): Error: undefined identifier strcmp

This is only a partial fix for the Unittest build not working. The other error I have not been able to track down the cause... I'm slightly leaning toward the attributes being assigned "pure nothrow @nogc @safe"

luad/stack.d(183,3): Error: static assert: "No Lua type defined for lua_State*"
luad/stack.d(224,23): instantiated from here: luaTypeOf!(lua_State*)
luad/stack.d(378,10): instantiated from here: getValue!(lua_State*, argumentTypeMismatch)
luad/conversions/functions.d(215,13): instantiated from here: getArgument!(extern (C) int function(lua_State*) pure nothrow @nogc @safe, 0)
luad/conversions/functions.d(248,23): ... (1 instantiations, -v to show) ...
luad/stack.d(133,15): instantiated from here: pushFunction!(extern (C) int function(lua_State*) pure nothrow @nogc @safe)
luad/stack.d(626,11): instantiated from here: pushValue!(extern (C) int function(lua_State*) pure nothrow @nogc @safe)

Rather than accepting the C function as a LuaFunction, it is trying to convert the lua_state* argument. See: #112