ANDROID: netfilter: nf_nat: remove static from nf_nat_ipv4_fn

Exporting a static function breaks the ABI toolchain when CFI is
enabled. Remove the static identifier to fix the issue.

Bug: 145210207
Change-Id: Ib23d594ae05718dcf119b8ed0b5a8251aee2561c
Signed-off-by: Sami Tolvanen <samitolvanen@google.com>