Fix trace.h

It looks like the variable "pc" is defined. At least the current code always
failed on me stating that "pc" is already defined somewhere else.

Let's use _pc instead, because that doesn't collide.

Is this the right approach? Does it break on 440 too? If not, why not?

Signed-off-by: Alexander Graf <agraf@suse.de>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>