expr: log: fix missing \0 when sending log prefix to kernel

expr: log: fix missing \0 when sending log prefix to kernel

If I run my automatic unit test for libnftables, It shows:
"ERROR: Expr NFT_EXPR_LOG_PREFIX mismatches"
a: test
b: test\ 6
       ^^
       garbage

The problem was in nft_rule_expr_log_build function. With
the attached patch, we use mnl_attr_put_strz() instead of
mnl_attr_put_str() as in other functions in the library.

Signed-off-by: Ana Rey <anarey@gmail.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>