MEDIUM: pools: call malloc_trim() from pool_gc()

If available it definitely makes sense to call it since it's also
called when stopping to reclaim the maximum possible memory.
diff --git a/src/pool.c b/src/pool.c
index 321f8bc..73082e8 100644
--- a/src/pool.c
+++ b/src/pool.c
@@ -303,6 +303,10 @@
 
 	if (!isolated)
 		thread_release();
+
+#if defined(HA_HAVE_MALLOC_TRIM)
+	malloc_trim(0);
+#endif
 }
 
 #else /* CONFIG_HAP_LOCKLESS_POOLS */