gbridge: Fix potential build issues

The gbridge.h header is using element defined in stdlib but
it doesn't include it which every include to gbridge.h
must be preceded by an include to stdlib.h.
Include stdlib.h from gbridge.h.

Signed-off-by: Alexandre Bailon <abailon@baylibre.com>

gbridge.h

@@ -21,6 +21,7 @@
 
 #include <config.h>
 #include <stdint.h>
+#include <stdlib.h>
 #include <linux/types.h>
 #include <sys/queue.h>