[submodules]: update sonic-linux-kernel

* 493ab6b 2018-10-01 | [Kernel] fix kernel build errors (#65) (origin/201807) [Ying Xie]
* 6a58489 2018-10-01 | uart: fix race between uart_put_char() and uart_shutdown() (#64) [lguohan]

Signed-off-by: Guohan Lu <gulv@microsoft.com>