Compilation fix for Apple clang-1700.0.13.5 (using C++ compile)

[johnhe4]

#3480

Issue when compiling C++, even though all code is valid C. Consider test.c:

int main()
{
   enum lws_write_protocol{ LWS_WRITE_CONTINUATION = 2, LWS_WRITE_NO_FIN = 0x40 };
   enum lws_write_protocol r = LWS_WRITE_CONTINUATION;
   r |= LWS_WRITE_NO_FIN;
   (void)r;
}

Compile with clang -x c++ test.c to see the issue.