Fix compiler errors in parser (#4612)

src/idl_parser.cpp: In member function 'flatbuffers::CheckedError flatbuffers::Parser::ParseHexNum(int, uint64_t*)':
src/idl_parser.cpp:220:62: error: type qualifiers ignored on cast result type [-Werror=ignored-qualifiers]
     if (!isxdigit(static_cast<const unsigned char>(cursor_[i])))
                                                              ^
src/idl_parser.cpp: In member function 'flatbuffers::CheckedError flatbuffers::Parser::Next()':
src/idl_parser.cpp:260:62: error: type qualifiers ignored on cast result type [-Werror=ignored-qualifiers]
         if(!isdigit(static_cast<const unsigned char>(*cursor_))) return NoError();
                                                              ^
cc1plus: all warnings being treated as errors