BUILD: wurfl: build fix for 1.9/2.0 code base

This applies the required changes for the new buffer API that came in 1.9.
This patch must be backported to 1.9.
diff --git a/src/wurfl.c b/src/wurfl.c
index 5c490f7..e319047 100644
--- a/src/wurfl.c
+++ b/src/wurfl.c
@@ -783,7 +783,7 @@
 	msg = &smp->strm->txn->req;
 	ctx.idx = 0;
 
-	if (http_find_full_header2(header_name, strlen(header_name), msg->chn->buf->p, idx, &ctx) == 0)
+	if (http_find_full_header2(header_name, strlen(header_name), ci_head(msg->chn), idx, &ctx) == 0)
 		return 0;
 
 	if (header_len > ctx.vlen)