build: cleanup includes of <gmodule.h>

<gmodule.h> is implicitly included by <gio/gio.h> which is available
everywhere. For that reason, we would not have to include this header
at all. However, it is recommended to explicitly include <gmodule.h>
where needed.

So, include it where needed -- if <gio/gio.h> wouldn't be there --
and drop it from where it is not needed.

src/main-utils.c

@@ -31,7 +31,6 @@
 
 #include <glib/gstdio.h>
 #include <glib-unix.h>
-#include <gmodule.h>
 
 #include "main-utils.h"
 #include "NetworkManagerUtils.h"