Move GStrv typedef to glib.h instead of gobject.h

GStrv was historically only needed for the boxed G_TYPE_STRV,
but it is now useful for g_auto(GStrv) as well. This is not
an ABI change.

https://bugzilla.gnome.org/show_bug.cgi?id=755355

glib/gstrfuncs.c

@@ -2458,6 +2458,13 @@ g_strsplit_set (const gchar *string,
   return result;
 }
 
+/**
+ * GStrv:
+ *
+ * A typedef alias for gchar**. This is mostly useful when used together with
+ * g_auto().
+ */
+
 /**
  * g_strfreev:
  * @str_array: a %NULL-terminated array of strings to free