src/main.c: Moved local prototype for global function to header 'funcs.h'

It is a bad idea to declare the prototype of an external function
in a file as it won't allow the compiler to cross-check it.

src/startup.c

@@ -56,6 +56,7 @@
 #include "actions.h"
 #include "client.h"
 #include "main.h"
+#include "funcs.h"
 #include "dock.h"
 #include "workspace.h"
 #include "keybind.h"