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)
Status Job ID Name Coverage
  Docker Image
passed #8001907
debian-buster

00:00:13

 
  Build And Test
passed #8001908
autotools

00:05:33

passed #8001909
meson

00:01:57

passed #8001910
meson-noglamor

00:01:46