checkpatch: add %pt as a valid vsprintf extension

checkpatch: add %pt as a valid vsprintf extension

Commit 4d42c44727a0 ("lib/vsprintf: Print time and date in human
readable format via %pt") introduced a new extension, %pt.

Add it in the list of valid extensions.

Link: http://lkml.kernel.org/r/20190314203719.29130-1-alexandre.belloni@bootlin.com
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Cc: Joe Perches <joe@perches.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>