secmark: Add libip6t_SECMARK

This patch adds the shared library module for the SECMARK target (IPv6).

Signed-off-by: James Morris <jmorris@namei.org>
diff --git a/extensions/Makefile b/extensions/Makefile
index 678bedc..24cdeb0 100644
--- a/extensions/Makefile
+++ b/extensions/Makefile
@@ -10,7 +10,7 @@
 
 ifeq ($(DO_SELINUX), 1)
 PF_EXT_SE_SLIB:=SECMARK
-PF6_EXT_SE_SLIB:=
+PF6_EXT_SE_SLIB:=SECMARK
 endif
 
 # Optionals