libweston: Properly namespace modules entrypoint

Use different functions so we cannot load a libweston common module in
weston directly or the other way around.

Signed-off-by: Quentin Glidic <sardemff7+git@sardemff7.net>
Reviewed-by: Daniel Stone <daniels@collabora.com>

fullscreen-shell/fullscreen-shell.c

@@ -34,6 +34,7 @@
 #include <assert.h>
 
 #include "compositor.h"
+#include "compositor/weston.h"
 #include "fullscreen-shell-unstable-v1-server-protocol.h"
 #include "shared/helpers.h"