BUILD: vars: need to include xxhash

It's needed for XXH3(), and it used to get it through other includes.
diff --git a/src/vars.c b/src/vars.c
index 37ff5f7..087bd1f 100644
--- a/src/vars.c
+++ b/src/vars.c
@@ -18,6 +18,7 @@
 #include <haproxy/tcpcheck.h>
 #include <haproxy/tools.h>
 #include <haproxy/vars.h>
+#include <haproxy/xxhash.h>
 
 
 /* This contains a pool of struct vars */