rhashtable: fix missing header

Fixup below build error:

include/linux/rhashtable.h: At top level:
include/linux/rhashtable.h:118:34: error: field ‘mutex’ has incomplete type
Signed-off-by: NYing Xue <ying.xue@windriver.com>
Acked-by: NThomas Graf <tgraf@suug.ch>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

include/linux/rhashtable.h

@@ -20,6 +20,7 @@
 
 #include <linux/list_nulls.h>
 #include <linux/workqueue.h>
+#include <linux/mutex.h>
 
 /*
  * The end of the chain is marked with a special nulls marks which has