MAINTAINERS: add Q: entry to kselftest for patchwork project

MAINTAINERS: add Q: entry to kselftest for patchwork project

A new patchwork project is created to track kselftest patches.  Update
the kselftest entry in the MAINTAINERS file adding 'Q:' entry:

  https://patchwork.kernel.org/project/linux-kselftest/list/

Link: http://lkml.kernel.org/r/20180515164427.12201-1-shuah@kernel.org
Signed-off-by: Shuah Khan (Samsung OSG) <shuah@kernel.org>
Cc: David S. Miller <davem@davemloft.net>
Cc: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Linus Walleij <linus.walleij@linaro.org>
Cc: Randy Dunlap <rdunlap@infradead.org>
Cc: Joe Perches <joe@perches.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>