Fix build breakage on Linux.

llvm-svn: 302295
diff --git a/lld/lib/Core/TaskGroup.cpp b/lld/lib/Core/TaskGroup.cpp
index aa20a0e..d4de48c 100644
--- a/lld/lib/Core/TaskGroup.cpp
+++ b/lld/lib/Core/TaskGroup.cpp
@@ -12,6 +12,7 @@
 
 #include <atomic>
 #include <stack>
+#include <thread>
 
 #if defined(_MSC_VER) && LLVM_ENABLE_THREADS
 #include <concrt.h>