Make sure os mutex define is visible in mutex.c

Signed-off-by: Jens Axboe <jens.axboe@oracle.com>
diff --git a/mutex.c b/mutex.c
index 5beaa08..1538f62 100644
--- a/mutex.c
+++ b/mutex.c
@@ -9,6 +9,7 @@
 #include "log.h"
 #include "mutex.h"
 #include "arch/arch.h"
+#include "os/os.h"
 
 void fio_mutex_remove(struct fio_mutex *mutex)
 {