Move declaration of max_user_call_depth to header

Move declaration of max_user_call_depth to header

Also removes an unnecessary declaration of cmdlist in cli-cmds.c.
I don't understand why it is there, the definition of cmdlist is
at the top of the same file.

gdb/ChangeLog:

2019-10-12  Christian Biesinger  <cbiesinger@google.com>

	* cli/cli-cmds.c (max_user_call_depth): Move comment to header.
	(show_user): Remove declaration of cmdlist.
	* cli/cli-cmds.h (max_user_call_depth): Declare.
	* cli/cli-script.c (execute_user_command): Remove declaration
	of max_user_call_depth.