lib/sign: allow to build with glib version less than 2.44

Ubuntu 14.04 uses glib-2.40 which have no some shiny macroses
for interface declaration.

Signed-off-by: Denis Pynkin <denis.pynkin@collabora.com>

src/libostree/ostree.h

@@ -40,5 +40,6 @@
 #include <ostree-repo-finder-mount.h>
 #include <ostree-repo-finder-override.h>
 #include <ostree-kernel-args.h>
+#include <ostree-sign.h>
 #include <ostree-autocleanups.h>
 #include <ostree-version.h>