Ehooks: Further optimize default merge case.

This avoids the cost of an iealloc in cases where the user uses the default
merge hook without using the default extent hooks.

src/ehooks.c

@@ -242,7 +242,7 @@ ehooks_default_merge_impl(tsdn_t *tsdn, void *addr_a, bool head_a, void *addr_b,
 	return false;
 }
 
-static bool
+bool
 ehooks_default_merge(extent_hooks_t *extent_hooks, void *addr_a, size_t size_a,
     void *addr_b, size_t size_b, bool committed, unsigned arena_ind) {
 	tsdn_t *tsdn = tsdn_fetch();