MINOR: vars: replace static functions with global ones

The OpenTracing filter uses several internal HAProxy functions to work
with variables and therefore requires two static local HAProxy functions,
var_accounting_diff() and var_clear(), to be declared global.

In fact, the var_clear() function was not originally defined as static,
but it lacked a declaration.
diff --git a/src/vars.c b/src/vars.c
index b847b5d..e328999 100644
--- a/src/vars.c
+++ b/src/vars.c
@@ -65,7 +65,7 @@
 /* This function adds or remove memory size from the accounting. The inner
  * pointers may be null when setting the outer ones only.
  */
-static void var_accounting_diff(struct vars *vars, struct session *sess, struct stream *strm, int size)
+void var_accounting_diff(struct vars *vars, struct session *sess, struct stream *strm, int size)
 {
 	switch (vars->scope) {
 	case SCOPE_REQ: