Fix compile warning on OSX

Signed-off-by: Jens Axboe <jaxboe@fusionio.com>
diff --git a/os/os-mac.h b/os/os-mac.h
index 0d3cae3..06ea462 100644
--- a/os/os-mac.h
+++ b/os/os-mac.h
@@ -8,6 +8,7 @@
 #include <sys/time.h>
 #include <unistd.h>
 #include <signal.h>
+#include <mach/mach_init.h>
 
 #include "../file.h"