Make sure to #include <atomic> when using std::atomic.

llvm-svn: 218148
diff --git a/lldb/source/lldb-log.cpp b/lldb/source/lldb-log.cpp
index 49c43af..463de13 100644
--- a/lldb/source/lldb-log.cpp
+++ b/lldb/source/lldb-log.cpp
@@ -11,6 +11,7 @@
 
 // C Includes
 // C++ Includes
+#include <atomic>
 // Other libraries and framework includes
 // Project includes
 #include "lldb/Interpreter/Args.h"