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/ofono/calls-ofono-provider.c

@@ -28,7 +28,7 @@
 #include "calls-provider.h"
 #include "calls-ofono-origin.h"
 #include "calls-message-source.h"
-#include "util.h"
+#include "calls-util.h"
 
 #include <libgdbofono/gdbo-manager.h>
 #include <libgdbofono/gdbo-modem.h>