tools: ynl: fix header path for nfsd

The makefile dependency is trying to include the wrong header:

<command-line>: fatal error: ../../../../include/uapi//linux/nfsd.h: No such file or directory

The guard also looks wrong.

Fixes: f14122b2

 ("tools: ynl: Add source files for nfsd netlink protocol")
Reviewed-by: Chuck Lever <chuck.lever@oracle.com>
Link: https://lore.kernel.org/r/20231123030624.1611925-1-kuba@kernel.org


Signed-off-by: Jakub Kicinski <kuba@kernel.org>