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>
diff --git a/tests/surface-screenshot.c b/tests/surface-screenshot.c
index 332b5e3..6ff2bfc 100644
--- a/tests/surface-screenshot.c
+++ b/tests/surface-screenshot.c
@@ -34,6 +34,7 @@
 #include <linux/input.h>
 
 #include "compositor.h"
+#include "compositor/weston.h"
 #include "file-util.h"
 
 static char *