shared/platform: include weston-egl-ext.h only if ENABLE_EGL is defined

Including `weston-egl-ext.h` causes compilation failure for configurations
with EGL disabled.

Verified with `--disable-egl`, `--disable-x11-compositor`
and `--disable-drm-compositor`.

Signed-off-by: Krzysztof Konopko <kris@youview.com>
Reviewed-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>

shared/platform.h

@@ -33,9 +33,9 @@
 #include <wayland-egl.h>
 #include <EGL/egl.h>
 #include <EGL/eglext.h>
-#endif
 
 #include "weston-egl-ext.h"
+#endif
 
 #ifdef  __cplusplus
 extern "C" {