linux: Add the missing lss dependency.

linux: Add the missing lss dependency.

Chromium ships with it in its code base, but since libchromiumcontent
doesn't contain it in the distribution, we need to include it herer to
make breakpad_client compile.

Note that we put the header under `vendor/third_party/lss` instead of
checking out the repository under `vendor`, because we don't want to
patch breakpad.