un-constify struct sess for varnish2

For varnish2 itself, we could keep the const qualifyer, but we use
the same interface for varnish3 and the varnish3 vmod interface
mandates an un-const argument. Special-casing this for varnish2
has no actual benefit.

src/dcs_varnish2.c

@@ -33,7 +33,7 @@
 #include "dcs_varnish.c"
 
 void
-dcs_varnish2_classify_hdrs(const struct sess *sp) {
+dcs_varnish2_classify_hdrs(struct sess *sp) {
 	const int e = dcs_varnish_classify(sp);
 	const int t = dcs_match_type_id(e);