gdb: make get_dyn_prop a method of struct type

gdb: make get_dyn_prop a method of struct type

Move get_dyn_prop, currently a free function, to be a method on struct
type.

gdb/ChangeLog:

	* gdbtypes.h (struct type) <get_dyn_prop>: New method.
	(get_dyn_prop): Remove.  Update all users to use
	type::dyn_prop.
	* gdbtypes.c (get_dyn_prop): Rename to...
	(type::dyn_prop): ... this.