make print-extensions doesn't show libxt_* extensions

In extensions/Makefile the variable PFX_EXT_SLIB_OPTS is not appended to
OPTIONALS, therefor 'make print-extensions' doesn't show any optional
libxt_* extension.

Sebastian Claßen <sebastian.classen@freenet.ag>
diff --git a/extensions/Makefile b/extensions/Makefile
index 93ead51..5af234e 100644
--- a/extensions/Makefile
+++ b/extensions/Makefile
@@ -85,6 +85,7 @@
 PF6_EXT_SLIB+=$(PF6_EXT_SLIB_OPTS)
 PFX_EXT_SLIB+=$(PFX_EXT_SLIB_OPTS)
 
+OPTIONALS+=$(patsubst %,XT:%,$(PFX_EXT_SLIB_OPTS))
 OPTIONALS+=$(patsubst %,IPv4:%,$(PF_EXT_SLIB_OPTS))
 OPTIONALS+=$(patsubst %,IPv6:%,$(PF6_EXT_SLIB_OPTS))