IB/mlx4: Include linux/mutex.h from mlx4_ib.h

mlx4_ib.h uses struct mutex, so although <linux/mutex.h> seems to be
pulled in indirectly by one of the headers it includes, the right
thing is to include <linux/mutex.h> directly.
Signed-off-by: Michael S. Tsirkin <mst@dev.mellanox.co.il>
Signed-off-by: Roland Dreier <rolandd@cisco.com>

drivers/infiniband/hw/mlx4/mlx4_ib.h

@@ -35,6 +35,7 @@
 
 #include <linux/compiler.h>
 #include <linux/list.h>
+#include <linux/mutex.h>
 
 #include <rdma/ib_verbs.h>
 #include <rdma/ib_umem.h>