libuafs: Add $(XCFLAGS) to CFLAGS on all platforms

libuafs: Add $(XCFLAGS) to CFLAGS on all platforms

Only darwin had $(XCFLAGS) in the libuafs build for some reason. Add
it to all platforms, so they get e.g. -Wall if so configured.

Change-Id: Ifab39af3abf087932feb1b9085ccb26baaec3164
Reviewed-on: http://gerrit.openafs.org/1745
Reviewed-by: Derrick Brashear <shadow@dementia.org>
Tested-by: Derrick Brashear <shadow@dementia.org>