.gitignore: ignore ASN.1 auto generated files

when build kernel with default configure, files:

generatenet/ipv4/netfilter/nf_nat_snmp_basic-asn1.c
net/ipv4/netfilter/nf_nat_snmp_basic-asn1.h

will be automatically generated by ASN.1 compiler, so
No need to track them in git, it's better to ignore them.

Signed-off-by: Zhu Lingshan <lszhu@suse.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>