tools: ynl-gen: use uapi header name for the header guard

Chuck points out that we should use the uapi-header property
when generating the guard. Otherwise we may generate the same
guard as another file in the tree.

Tested-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Reviewed-by: Przemek Kitszel <przemyslaw.kitszel@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>