xfrm4: Add IPsec protocol multiplexer

This patch add an IPsec protocol multiplexer. With this
it is possible to add alternative protocol handlers as
needed for IPsec virtual tunnel interfaces.

Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>
diff --git a/net/ipv4/Makefile b/net/ipv4/Makefile
index f8c49ce..f032688 100644
--- a/net/ipv4/Makefile
+++ b/net/ipv4/Makefile
@@ -55,4 +55,4 @@
 obj-$(CONFIG_NETLABEL) += cipso_ipv4.o
 
 obj-$(CONFIG_XFRM) += xfrm4_policy.o xfrm4_state.o xfrm4_input.o \
-		      xfrm4_output.o
+		      xfrm4_output.o xfrm4_protocol.o