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-call-selector-item.c

@@ -25,7 +25,7 @@
 #include "calls-call-selector-item.h"
 #include "calls-call.h"
 #include "calls-ui-call-data.h"
-#include "util.h"
+#include "calls-util.h"
 
 #include <glib/gi18n.h>
 #include <glib-object.h>