Add Atomic.cpp to the CMake build system.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@72202 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/lib/System/CMakeLists.txt b/lib/System/CMakeLists.txt
index 4b9ce5d..5415dd6 100644
--- a/lib/System/CMakeLists.txt
+++ b/lib/System/CMakeLists.txt
@@ -1,5 +1,6 @@
 add_llvm_library(LLVMSystem
   Alarm.cpp
+  Atomic.cpp
   Disassembler.cpp
   DynamicLibrary.cpp
   Host.cpp