xwayland: Fix build warning without glamor

Building Xwayland without glamor support would raise a warning at build
time:

  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>

hw/xwayland/xwayland.c

@@ -977,7 +977,9 @@ xwl_screen_init(ScreenPtr pScreen, int argc, char **argv)
     struct xwl_screen *xwl_screen;
     Pixel red_mask, blue_mask, green_mask;
     int ret, bpc, green_bpc, i;
+#ifdef XWL_HAS_GLAMOR
     Bool use_eglstreams = FALSE;
+#endif
 
     xwl_screen = calloc(1, sizeof *xwl_screen);
     if (xwl_screen == NULL)