tools: ynl: make user space policies const

Dan, who's working on C++ YNL, pointed out that the C code
does not make policies const. Sprinkle some 'const's around.

Reported-by: Dan Melnic <dmm@meta.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Reviewed-by: Donald Hunter <donald.hunter@gmail.com>
Reviewed-by: Nicolas Dichtel <nicolas.dichtel@6wind.com>
Signed-off-by: David S. Miller <davem@davemloft.net>