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-sign.c

@@ -47,6 +47,8 @@
 #include "ostree-sign-ed25519.h"
 #endif
 
+#include "ostree-autocleanups.h"
+
 #undef G_LOG_DOMAIN
 #define G_LOG_DOMAIN "OSTreeSign"