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>