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-origin.c

@@ -24,7 +24,7 @@
 
 #include "calls-origin.h"
 #include "calls-message-source.h"
-#include "util.h"
+#include "calls-util.h"
 
 /**
  * SECTION:calls-origin