Fix return type mismatches between header and implementation

nautilus_file_get_uid and nautilus_file_get_gid were declared to return a "uid_t" or "gid_t" in the header, but a "const uid_t" or "const gid_t" in the implementation.

This mismatch caused the build to fail with clang 15.x (and will probably cause errors in a future gcc version as well).

Since there's no difference between returning an int and a const int, use the simpler version in both contexts.