Move frame_id_build_unavailable_stack static

frame_id_build_unavailable_stack is only used in frame.c, so make it static.

gdb:

2017-07-28  Yao Qi  <yao.qi@linaro.org>

	* frame.c (frame_id_build_unavailable_stack): Move it up.
	* frame.h (frame_id_build_unavailable_stack): Remove the declaration.