.gitignore: Ignore src/third_party/khronos/CL.

We started checking out this directory with Crosswalk PR#2426, but since
we do not have it in .gitignore the directory ends up being removed
every time we run gclient sync.
Merge pull request #195 from rakuco/ignore-khronos-CL

.gitignore: Ignore src/third_party/khronos/CL.