.gitignore: Add techpack directory to gitignore

The code under the techpack directory is maintained
in different repositories, and thus, should be ignored
by the kernel git repository, so add the techpack
directory to the gitignore.

Change-Id: Id8f737e3ddf36ca983b81e62bf7b881d61916353
Signed-off-by: Isaac J. Manjarres <isaacm@codeaurora.org>