Cycles: Use lock in the memory statistics

CPU rendering is allowed to allocate memory from multiple threads,
which means statistics need to be avare of this.

intern/cycles/CMakeLists.txt

@@ -163,6 +163,10 @@ include_directories(
 	${OPENEXR_INCLUDE_DIRS}
 )
 
+# TODO(sergey): Adjust so standalone repository is also happy.
+include_directories(
+	../atomic
+)
 
 # Warnings
 if(CMAKE_COMPILER_IS_GNUCXX)