clang-tidy: Add missing atomic header

Change-Id: Ibb143e8ed9cacadb2a1009d1f71fff41cf5c90b9
Signed-off-by: Willy Tu <wltu@google.com>
diff --git a/src/ipmiblob/ipmi_handler.cpp b/src/ipmiblob/ipmi_handler.cpp
index 90d3510..b51718a 100644
--- a/src/ipmiblob/ipmi_handler.cpp
+++ b/src/ipmiblob/ipmi_handler.cpp
@@ -24,6 +24,7 @@
 #include <sys/ioctl.h>
 
 #include <array>
+#include <atomic>
 #include <cstdint>
 #include <cstring>
 #include <memory>