ipv4: Make fib_encap_match static

Make fib_encap_match() static as it isn't used outside the file.

Signed-off-by: Ying Xue <ying.xue@windriver.com>
Reviewed-by: Jiri Benc <jbenc@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>