xwayland: Fix build without glamor

When building Xwayland without glamor support enabled using automake,
the build would fail at link time trying to find `glamor_block_handler`:

  /usr/bin/ld: xwayland-glx.o: in function `egl_drawable_wait_x':
  hw/xwayland/xwayland-glx.c:102: undefined reference to
  `glamor_block_handler'

Make sure we don't try to build `xwayland-glx.c` without glamor in the
Xwayland Makefile.

Note: Meson build is fine because it's already build only with glamor
enabled.

Fixes: commit 84692415 - "xwayland: Add EGL-backed GLX provider"
Signed-off-by: Olivier Fourdan <ofourdan@redhat.com>
3 jobs for xwayland-glamor-automake in 5 minutes and 49 seconds (queued for 1 second)
Status Job ID Name Coverage
  Docker Image
passed #460753
debian-testing

00:00:34

 
  Build And Test
passed #460754
autotools-build-and-test

00:05:14

passed #460755
meson-build-and-test

00:01:30