[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.

llvm/lib/Debuginfod/Debuginfod.cpp

@@ -40,6 +40,7 @@
 #include "llvm/Support/xxhash.h"
 
 #include <atomic>
+#include <thread>
 
 namespace llvm {
 static std::string uniqueKey(llvm::StringRef S) { return utostr(xxHash64(S)); }