Fix hash_map.h on non-GNU C++ standard library implementations.

This fixes the massive deprecation warnings reported by @rburchell.

This should already be fixed in master as this header was replaced when we removed support for __gnu_cxx::hash_map (see [include/compat.h](https://github.com/inspircd/inspircd/blob/e6d245a3e80c0e0c05476e5bd9c45a77e16fecec/include/compat.h#L24)).