tools: ynltool: remove -lmnl from link flags
The libmnl dependency has been removed from libynl back in
commit 73395b43819b ("tools: ynl: remove the libmnl dependency")
Remove it from the ynltool Makefile.
Reviewed-by: Donald Hunter <donald.hunter@gmail.com>
Link: https://patch.msgid.link/20251115225508.1000072-1-kuba@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>