Instead of declaring debug_threads and using_threads in several
.c files, declare them in debug.h.

gdb/gdbserver/ChangeLog:

2019-09-20  Christian Biesinger  <cbiesinger@google.com>

	* debug.c (debug_threads): Remove comment in favor of the header.
	* debug.h (using_threads): Add declaration.
	(debug_threads): Add comment.
	* linux-aarch64-low.c: Include debug.h and remove declaration of
	debug_threads.
	* nto-low.c: Likewise.
	* remote-utils.c: Likewise.
	* thread-db.c: Likewise.