build: Rename util.{c,h} to calls-util.{c,h}

"util" is a very generic name. This guards against accidentally
including similarly named headers from elsewhere.

src/calls-main-window.c

@@ -32,7 +32,7 @@
 #include "calls-in-app-notification.h"
 #include "calls-manager.h"
 #include "calls-config.h"
-#include "util.h"
+#include "calls-util.h"
 #include "version.h"
 
 #include <glib/gi18n.h>