Fix OSX build

os.h needs to include sys/socket.h.

Signed-off-by: Jens Axboe <axboe@kernel.dk>
diff --git a/os/os.h b/os/os.h
index 1218815..d6ce9a7 100644
--- a/os/os.h
+++ b/os/os.h
@@ -2,6 +2,7 @@
 #define FIO_OS_H
 
 #include <sys/types.h>
+#include <sys/socket.h>
 #include <pthread.h>
 #include <unistd.h>
 #include <stdlib.h>