xwayland: Fix build warning without glamor

Building Xwayland without glamor support would raise a warning at build

  xwayland.c: In function ‘xwl_screen_init’:
  xwayland.c:980:10: warning: unused variable ‘use_eglstreams’
    980 |     Bool use_eglstreams = FALSE;
        |          ^~~~~~~~~~~~~~

When building without glamor support, we cannot have EGL Streams support
either, the two being related. So we do not need to declare the variable
`use_eglstreams` if glamor is not enabled.

Signed-off-by: Olivier Fourdan <ofourdan@redhat.com>
Reviewed-by: Michel Dänzer <michel.daenzer@amd.com>
3 jobs for xwayland-eglstream-without-glamor in 6 minutes and 5 seconds (queued for 22 seconds)