mi/mi-cmd-stack.c|frame filters: print_values <-> ext_lang_frame_args

mi/mi-cmd-stack.c|frame filters: print_values <-> ext_lang_frame_args

The enums are value compatible by design, but building in C++ mode trips
on them, like:

  ...
  gdb/mi/mi-cmd-stack.c:363:34: error: cannot convert ‘print_values’ to ‘ext_lang_frame_args’ for argument ‘3’ to ‘ext_lang_bt_status apply_ext_lang_frame_filter(frame_info*, int, ext_lang_frame_args, ui_out*, int, int)’
  ...

Fix this by adding a helper function.

gdb/ChangeLog:
2015-02-27  Pedro Alves  <palves@redhat.com>

	* mi/mi-cmd-stack.c (mi_apply_ext_lang_frame_filter): New
	function.
	(mi_cmd_stack_list_locals, mi_cmd_stack_list_args)
	(mi_cmd_stack_list_variables): Use it.