tracing: Mention trace_clock=global when warning about unstable clocks

Mention the alternative of adding trace_clock=global to the kernel
command line when we detect that we've used an unstable clock across a
suspend/resume cycle.

Link: http://lkml.kernel.org/r/20180330150132.16903-2-chris@chris-wilson.co.uk



Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>