Add destroy_global_config.  Call it on shutdown

This is for symmetry with init_global_config.
diff --git a/libltrace.c b/libltrace.c
index b69a0c9..54eacb2 100644
--- a/libltrace.c
+++ b/libltrace.c
@@ -91,6 +91,7 @@
 {
 	if (options.summary) {
 		show_summary();
+		destroy_global_config();
 	}
 	if (options.output) {
 		fclose(options.output);