wayland-egl: Define WL_EGL_PLATFORM in wayland-egl.h

This indicates that we're using the wayland egl platform and will make
eglplatform.h typedef the native types correctly.

wayland/wayland-egl.h

@@ -30,6 +30,8 @@ extern "C" {
 
 #include <wayland-client.h>
 
+#define WL_EGL_PLATFORM 1
+
 struct wl_egl_display;
 struct wl_egl_window;
 struct wl_egl_pixmap;