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 8469241 - "xwayland: Add EGL-backed GLX provider"
Signed-off-by: Olivier Fourdan <ofourdan@redhat.com>

hw/xwayland/Makefile.am

@@ -21,13 +21,15 @@ Xwayland_SOURCES =				\
 	$(top_srcdir)/Xi/stubs.c		\
 	$(top_srcdir)/mi/miinitext.c
 
+if GLAMOR
 if GLX
 Xwayland_SOURCES += 				\
 	xwayland-glx.c
 Xwayland_CFLAGS +=				\
 	-I$(top_srcdir)/glx
 GLXVND_LIB = $(top_builddir)/glx/libglxvnd.la
-endif
+endif #GLX
+endif # GLAMOR
 
 Xwayland_LDADD =				\
 	$(glamor_lib)				\