gdb: Move get_frame_language from stack.c to frame.c.

gdb: Move get_frame_language from stack.c to frame.c.

The get_frame_language feels like it would be more at home in frame.c
rather than in stack.c, while the declaration, that is currently in
language.h can be moved into frame.h to match.

A couple of new includes are added, but otherwise no substantial change
here.

gdb/ChangeLog:

	* stack.c (get_frame_language): Moved ...
	* frame.c (get_frame_language): ... to here.
	* language.h (get_frame_language): Declaration moved to frame.h.
	* frame.h: Add language.h include, for language enum.
	(get_frame_language): Declaration moved from language.h.
	* language.c: Add frame.h include.
	* top.c: Add frame.h include.
	* symtab.h (struct obj_section): Declare.
	(struct cmd_list_element): Declare.