This constifies add_com_suppress_notification and fixes the one
caller.
gdb/ChangeLog
2017-11-07 Tom Tromey <tom@tromey.com>
* stack.c (select_frame_command): Constify.
* cli/cli-decode.c (add_com_suppress_notification): Constify.
* command.h (add_com_suppress_notification): Constify.