[LLVM][Debuginfod] Add missing thread include

One of our silent bots is currently failing:
https://lab.llvm.org/staging/#/builders/171/builds/169

With:
<...>/Debuginfod.cpp:298:23: error: no type named 'sleep_for' in namespace 'std::this_thread'
    std::this_thread::sleep_for(Interval);
    ~~~~~~~~~~~~~~~~~~^

Add missing thread include to that file,
which is what all the other users of sleep_for do.

I think we are seeing this now because we disabled
llvm threading for this builder. Maybe debuginfod should account
for that but that's for another time.