MINOR: stick-tables: export process_table_expire()

This handler can take quite some time as it deletes a large number of
entries under a lock, let's export it so that it's immediately visible
in "show profiling".
diff --git a/src/stick_table.c b/src/stick_table.c
index da07b8c..485f6f1 100644
--- a/src/stick_table.c
+++ b/src/stick_table.c
@@ -626,7 +626,7 @@
  * Task processing function to trash expired sticky sessions. A pointer to the
  * task itself is returned since it never dies.
  */
-static struct task *process_table_expire(struct task *task, void *context, unsigned short state)
+struct task *process_table_expire(struct task *task, void *context, unsigned short state)
 {
 	struct stktable *t = context;