[sonic-linux-kernel] Update submodule (#3573)

Debain maintainers took down the 3.16.68-2 kernel source, which broke the 201803 build. This patch downloads the files from Azure storage instead.

src/sonic-linux-kernel 9ccb78e...5ec0c26 (2):
[201803] Revert kernel upgrade; Download 3.16.68-2 kernel from Azure Storage
[201803] Upgrade kernel from 3.16.68-2 to 3.16.74-1

Note: Check builds failing due to saithrift issue, which is fixed here: #3576.

Author: jleveque