os.h should include headers it needs by itself

Signed-off-by: Jens Axboe <jens.axboe@oracle.com>
diff --git a/os/os.h b/os/os.h
index 9a48c60..d128f02 100644
--- a/os/os.h
+++ b/os/os.h
@@ -1,6 +1,9 @@
 #ifndef FIO_OS_H
 #define FIO_OS_H
 
+#include <sys/types.h>
+#include <unistd.h>
+
 #if defined(__linux__)
 #include "os-linux.h"
 #elif defined(__FreeBSD__)