audit: export symbol for use with xt_AUDIT

When xt_AUDIT is built as a module, modpost reports a problem.

	MODPOST 322 modules
	ERROR: "audit_enabled" [net/netfilter/x_tables.ko] undefined!
	WARNING: modpost: Found 1 section mismatch(es).

Cc: Thomas Graf <tgraf@redhat.com>
Signed-off-by: Jan Engelhardt <jengelh@medozas.de>