Remove unnecessary const qualifier

[avsej]

In file included from include/flatbuffers/flexbuffers.h:24,
                 from src/idl_gen_text.cpp:20:
include/flatbuffers/util.h: In function 'int flatbuffers::FromUTF8(const char**)':
include/flatbuffers/util.h:324:45: error: type qualifiers ignored on cast result type [-Werror=ignored-qualifiers]
   if ((static_cast<const unsigned char>(**in) << len) & 0x80) return -1;  // Bit after leading 1's must be 0.
                                             ^
cc1plus: all warnings being treated as errors
make[2]: *** [CMakeFiles/flatbuffers_shared.dir/build.make:92: CMakeFiles/flatbuffers_shared.dir/src/idl_gen_text.cpp.o] Error 1

[aardappel]

Thanks!