glamor: Fix build without GBM

The functions glamor_egl_fd_from_pixmap()/glamor_egl_fds_from_pixmap()
are not available without GBM support.

So if GBM is not available or too old, the code would fail to link
trying to find the references to those functions.

Make sure we skip that code when glamor is built without GBM.

Signed-off-by: Olivier Fourdan <ofourdan@redhat.com>

glamor/glamor.c

@@ -1003,6 +1003,7 @@ _glamor_fds_from_pixmap(ScreenPtr screen, PixmapPtr pixmap, int *fds,
                         uint32_t *strides, uint32_t *offsets,
                         CARD32 *size, uint64_t *modifier)
 {
+#ifdef GLAMOR_HAS_GBM
     glamor_pixmap_private *pixmap_priv = glamor_get_pixmap_private(pixmap);
     glamor_screen_private *glamor_priv =
         glamor_get_screen_private(pixmap->drawable.pScreen);
@@ -1030,6 +1031,7 @@ _glamor_fds_from_pixmap(ScreenPtr screen, PixmapPtr pixmap, int *fds,
     default:
         break;
     }
+#endif /* GLAMOR_HAS_GBM */
     return 0;
 }