builder-tool: Avoid duplicate path prefixes

It is enough to print the file in which the error
occurred once; GtkBuilder already makes that part
of its error message.

gtk/tools/gtk-builder-tool-validate.c

@@ -139,7 +139,7 @@ validate_file (const char *filename)
         }
       else
         {
-          g_printerr ("%s: %s\n", filename, error->message);
+          g_printerr ("%s\n", error->message);
           return FALSE;
         }
     }