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.

plugins/provider/mm/calls-mm-call.c

@@ -27,7 +27,7 @@
 #include "calls-mm-call.h"
 #include "calls-call.h"
 #include "calls-message-source.h"
-#include "util.h"
+#include "calls-util.h"
 
 #include <libmm-glib/libmm-glib.h>
 #include <glib/gi18n.h>