Makefile: handle also stable wayland-protocols

Makefile: handle also stable wayland-protocols

Fix the protostability function to handle stable protocol files
correctly. Stable protocol XML file names do not have 'stable' in their
name, nor do we want to write that in the prerequisite lists in the
Makefile.

Function 'protoname' does not need fixing, because for stable protocol
prerequisites, the sed pattern will not match, and it passes stem
through as is, which is correct.

Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>
Reviewed-by: Bryce Harrington <bryce@osg.samsung.com>