x11/keymap: Silence deprecation warnings around a fallback

x11/keymap: Silence deprecation warnings around a fallback

We still use XKeycodeToKeysym() in a fallback path in case we're not
running on a decent enough system; XKeycodeToKeysym() is deprecated as
of version 1.12 of the X server, but since I don't want to copy a bunch
of code from GDK or, god forbid, from Xlib, for a fallback path, it's
probably more reasonable to just silence the compiler warnings - at
least until we can drop all the X compatibility crap, and just use
modern, or semi-modern, API.