build: separate version from the rest of the configuration data

This should make sure that we always have 'config.h' available and
should fix issues with flatpak build as seen in #257:

`fatal error: config.h: No such file or directory`

src/calls-application.c

@@ -36,6 +36,7 @@
 #include "calls-main-window.h"
 #include "calls-manager.h"
 #include "calls-application.h"
+#include "version.h"
 
 #include <glib/gi18n.h>
 #include <handy.h>