build: cleanup include of <config.h>

It is recomended to include <config.h> with angle brackets [1].

Note, that usually we don't include <config.h> directly, except in
two places we have to (because there we include conflicting libraries
that must be included before "nm-default.h").
In that case, define __CONFIG_H__ which is used as include guard around
<config.h> by "nm-default.h".

[1] https://www.gnu.org/software/autoconf/manual/autoconf-2.66/html_node/Configuration-Headers.html

src/dhcp-manager/nm-dhcp-dhclient.c

@@ -18,7 +18,8 @@
  * Copyright (C) 2005 - 2012 Red Hat, Inc.
  */
 
-#include "config.h"
+#include <config.h>
+#define __CONFIG_H__
 
 #define _XOPEN_SOURCE
 #include <time.h>