Add missing va_end()

Fixes https://github.com/behdad/harfbuzz/pull/74
diff --git a/util/options.cc b/util/options.cc
index 0adc179..5536820 100644
--- a/util/options.cc
+++ b/util/options.cc
@@ -55,6 +55,7 @@
   va_list vap;
   va_start (vap, format);
   msg = g_strdup_vprintf (format, vap);
+  va_end (vap);
   const char *prgname = g_get_prgname ();
   g_printerr ("%s: %s\n", prgname, msg);
   if (suggest_help)