You should be able to suppress this using
#pragma GCC system_header
But that applies for the rest of the file, and you cannot only use it in included files. So doesn't provide perfect scoping and may require some reshuffling / indirect inclusion of header files.
(But quite frankly, if you can't fix the header file to be standards-conforming, you might as well consider the whole header a system_header, which excludes it from producing most warnings.)
See https://gcc.gnu.org/onlinedocs/cpp/System-Headers.html
