Fix up complaints.h for namespace use

Fix up complaints.h for namespace use

If 'complaint' is used in a namespace context, it will fail because
'stop_whining' is only declared at the top level.  This patch fixes
this problem in a simple way, by moving the declaration of
'stop_whining' out of the macro and to the top-level.

gdb/ChangeLog
2020-04-08  Tom Tromey  <tromey@adacore.com>

	* complaints.h (stop_whining): Declare at top-level.
	(complaint): Don't declare stop_whining.