xfrm4: Remove duplicate semicolon

3328715e (xfrm4: Add IPsec protocol multiplexer) adds a
duplicate semicolon after the return-statement.

Although it has no negative impact, the second semicolon should be
removed.

Cc: Steffen Klassert <steffen.klassert@secunet.com>
Cc: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: Christoph Paasch <christoph.paasch@uclouvain.be>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>