test: avoid unused warning for test-display

meson -Dwith_x11=yes -Dwith_wayland=no -Dwith_drm=no -Dwith_egl=no
-Dwith_glx=no buildir

generate unused warnings.

tests/internal/test-display.c

@@ -205,7 +205,7 @@ int
 main (int argc, char *argv[])
 {
   GstVaapiDisplay *display;
-#if USE_X11 || USE_WAYLAND
+#if USE_GLX || USE_WAYLAND
   guint width, height;
   guint par_n, par_d;
 #endif