trivial: Don't include config.h in headers

This should be included by each .c file.  This fixes using libostree
from a "plain" project without config.h.

src/libostree/ostree-gpg-verifier.c

@@ -21,6 +21,8 @@
  * Author: Sjoerd Simons <sjoerd.simons@collabora.co.uk>
  */
 
+#include "config.h"
+
 #include "ostree-gpg-verifier.h"
 #include "otutil.h"
 

src/libostree/ostree-gpg-verifier.h

@@ -23,7 +23,6 @@
 
 //#pragma once
 
-#include "config.h"
 #include <glib-object.h>
 #include <gio/gio.h>
 

src/libostree/ostree-repo.h

@@ -22,7 +22,6 @@
 
 #pragma once
 
-#include "config.h"
 #include "ostree-core.h"
 #include "ostree-types.h"