parser_bison: release helper type string after parsing

parser_bison: release helper type string after parsing

==4060==ERROR: LeakSanitizer: detected memory leaks

Direct leak of 4 byte(s) in 1 object(s) allocated from:
    #0 0x7f637b64a810 in strdup (/usr/lib/x86_64-linux-gnu/libasan.so.5+0x3a810)
    #1 0x7f637b17766d in xstrdup /home/pablo/devel/scm/git-netfilter/nftables/src/utils.c:75
    #2 0x7f637b1ddce9 in nft_parse /home/pablo/devel/scm/git-netfilter/nftables/src/parser_bison.c:5792

Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>