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

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

Fixes: commit 84692415 - "xwayland: Add EGL-backed GLX provider"
Signed-off-by: Olivier Fourdan <ofourdan@redhat.com>
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