Don't mark GThread struct as deprecated

Having a definition of struct _GThread inside a deprecation
ifdef confuses gtk-doc into marking it as deprecated. Avoid this.

https://bugzilla.gnome.org//show_bug.cgi?id=735297

glib/deprecated/gthread.h

@@ -43,6 +43,8 @@ typedef enum
   G_THREAD_PRIORITY_URGENT
 } GThreadPriority;
 
+#endif
+
 struct  _GThread
 {
   /*< private >*/
@@ -52,6 +54,8 @@ struct  _GThread
   GThreadPriority priority;
 };
 
+#ifndef G_DISABLE_DEPRECATED
+
 typedef struct _GThreadFunctions GThreadFunctions;
 struct _GThreadFunctions
 {