gdb: Document C++17 build requirement.

We require C++17 to build for a while now:
https://sourceware.org/git/?p=binutils-gdb.git;a=commit;h=f74dc26792a0679e29db45e56367331ff48666d1

Reviewed-By: Lancelot Six <lancelot.six@amd.com>
1 file changed