tree-wide: Fix the build with old glib (Ubuntu Trusty etc.)

This regressed with <https://github.com/ostreedev/ostree/pull/1040>
but currently the Travis builds aren't gating.

Closes: #1051
Approved by: jlebon

src/libostree/ostree-mutable-tree.c

@@ -22,8 +22,8 @@
 
 #include "config.h"
 
-#include "ostree.h"
 #include "otutil.h"
+#include "ostree.h"
 
 /**
  * SECTION:ostree-mutable-tree