don't need that forward declaration of struct nameidata in dcache.h anymore

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

include/linux/dcache.h

@@ -11,7 +11,6 @@
 #include <linux/rcupdate.h>
 #include <linux/lockref.h>
 
-struct nameidata;
 struct path;
 struct vfsmount;