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/[email protected]
Signed-off-by: Alexandre Belloni <[email protected]>
Cc: Joe Perches <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>