backend-headless: fix build issue without gl-renderer

If the gl-renderer is disabled build fails with:
  ../libweston/backend-headless/headless.c:394:9: error:
  ‘EGL_PLATFORM_SURFACELESS_MESA’ undeclared (first use in this function)

Fix this by including shared/weston-egl-ext.h.

Signed-off-by: Stefan Agner <stefan@agner.ch>

libweston/backend-headless/headless.c

@@ -40,6 +40,7 @@
 #include "linux-explicit-synchronization.h"
 #include "pixman-renderer.h"
 #include "renderer-gl/gl-renderer.h"
+#include "shared/weston-egl-ext.h"
 #include "linux-dmabuf.h"
 #include "presentation-time-server-protocol.h"
 #include <libweston/windowed-output-api.h>