Two minor constifications

Two minor constifications

I noticed a couple of spots where a "char *" was used where a
"const char *" made more sense.  This patch fixes both of them.
Tested by rebuilding.

gdb/ChangeLog
2019-05-10  Tom Tromey  <tromey@adacore.com>

	* thread.c (print_thread_info): Make "requested_threads" const.
	* gdbthread.h (print_thread_info): Make "requested_threads"
	const.
	* ada-tasks.c (print_ada_task_info): Make "taskno_str" const.
	* ada-lang.h (print_ada_task_info): Make "taskno_str" const.