gobject: Deprecate GParameter

GParameter is a rarely used type and not introspectible.

https://bugzilla.gnome.org/show_bug.cgi?id=709865

gobject/gparam.h

@@ -267,6 +267,8 @@ struct _GParamSpecClass
  * 
  * The GParameter struct is an auxiliary structure used
  * to hand parameter name/value pairs to g_object_newv().
+ *
+ * Deprecated: 2.52: This type is not introspectable.
  */
 struct _GParameter /* auxiliary structure for _setv() variants */
 {