parser_bison: release parsed type and hook name strings

parser_bison: release parsed type and hook name strings

The scanner allocates memory for this, so release them given that we
don't attach them to any object.

==6277== 42 bytes in 6 blocks are definitely lost in loss record 2 of 4
==6277==    at 0x4C28C20: malloc (vg_replace_malloc.c:296)
==6277==    by 0x57AC9D9: strdup (strdup.c:42)
==6277==    by 0x41B82D: xstrdup (utils.c:64)
==6277==    by 0x41F510: nft_lex (scanner.l:511)
==6277==    by 0x427FD1: nft_parse (parser_bison.c:3690)
==6277==    by 0x4063AC: nft_run (main.c:231)
==6277==    by 0x40600C: main (main.c:361)

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