meson: Make sure XKM_OUTPUT_DIR has a trailing slash

RunXkbComp passes an invalid file path to xkbcomp if there's no
trailing slash.

Closes: xorg/xserver#1146
Suggested-by: Dylan Baker <dylan.c.baker@intel.com>
4 jobs for meson-XKM_OUTPUT_DIR-trailing-slash in 5 minutes and 46 seconds (queued for 1 second)