libweston: Migrate functions that operate on 'weston_spring'

Signed-off-by: Marius Vlad <marius.vlad@collabora.com>
diff --git a/libweston/zoom.c b/libweston/zoom.c
index 8d513c3..ac8f1f3 100644
--- a/libweston/zoom.c
+++ b/libweston/zoom.c
@@ -31,6 +31,7 @@
 #include <stdbool.h>
 
 #include <libweston/libweston.h>
+#include "libweston-internal.h"
 #include "text-cursor-position-server-protocol.h"
 #include "shared/helpers.h"