Fix warnings and symbol visibility from missing CFLAGS
diff --git a/compositor/shell.c b/compositor/shell.c
index 1dc2427..ab6218b 100644
--- a/compositor/shell.c
+++ b/compositor/shell.c
@@ -733,6 +733,9 @@
 {
 }
 
+int
+shell_init(struct wlsc_compositor *ec);
+
 WL_EXPORT int
 shell_init(struct wlsc_compositor *ec)
 {