mlx4_core: Include linux/mutex.h from mlx4.h

mlx4.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 to do 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/net/mlx4/mlx4.h

@@ -37,6 +37,7 @@
 #ifndef MLX4_H
 #define MLX4_H
 
+#include <linux/mutex.h>
 #include <linux/radix-tree.h>
 
 #include <linux/mlx4/device.h>