gprofng: avoid using `fallthrough' attributes

gprofng didn't build with gcc 6.3 due to the usage of __attribute__
((fallthrough)).  This patch uses /* FALLTHROUGH */ instead.

2022-03-15  Jose E. Marchesi  <jose.marchesi@oracle.com>

	* gprofng/src/gp-collect-app.cc (collect::check_args): Use
	fallthrough comment instead of attribute.