namei.h: include errno.h

This solves:

In file included from fs/ext3/symlink.c:20:0:
include/linux/namei.h: In function 'retry_estale':
include/linux/namei.h:114:19: error: 'ESTALE' undeclared (first use in this function)
Signed-off-by: Stephen Warren <swarren@wwwdotorg.org>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

include/linux/namei.h

@@ -2,6 +2,7 @@
 #define _LINUX_NAMEI_H
 
 #include <linux/dcache.h>
+#include <linux/errno.h>
 #include <linux/linkage.h>
 #include <linux/path.h>