netfilter: nf_nat_snmp_basic: add correct dependency to Makefile

nf_nat_snmp_basic_main.c includes a generated header, but the
necessary dependency is missing in Makefile. This could cause
build error in parallel building.

Remove a weird line, and add a correct one.

Fixes: cc2d5863 ("netfilter: nf_nat_snmp_basic: use asn1 decoder library")
Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>