Driver core: include linux/mutex.h from attribute_container.c

attribute_container.c uses DEFINE_MUTEX, so while
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>

drivers/base/attribute_container.c

@@ -18,6 +18,7 @@
 #include <linux/slab.h>
 #include <linux/list.h>
 #include <linux/module.h>
+#include <linux/mutex.h>
 
 #include "base.h"