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>
(cherry picked from commit 6c51818a)
3 jobs for xwayland-21.1-xkm_output_dir-trailing-slash in 1 minute and 53 seconds (queued for 1 second)