parser_bison: ensure all timeout policy names are released

parser_bison: ensure all timeout policy names are released

We need to add a custom destructor for this structure, it
contains the dynamically allocated names.

a:5:55-55: Error: syntax error, unexpected '}', expecting string
policy = { estabQisheestablished : 2m3s, cd : 2m3s, }

==562373==ERROR: LeakSanitizer: detected memory leaks

Indirect leak of 160 byte(s) in 2 object(s) allocated from:
    #1 0x5a565b in xmalloc src/utils.c:31:8
    #2 0x5a565b in xzalloc src/utils.c:70:8
    #3 0x3d9352 in nft_parse_bison_filename src/libnftables.c:520:8
[..]

Fixes: c7c94802679c ("src: add ct timeout support")
Signed-off-by: Florian Westphal <fw@strlen.de>