Add missing include

Author: jansvoboda11

llvm/lib/Target/ARM/ARMExpandPseudoInsts.cpp

@@ -27,6 +27,8 @@
 #include "llvm/MC/MCAsmInfo.h"
 #include "llvm/Support/Debug.h"
 
+#include <atomic>
+
 using namespace llvm;
 
 #define DEBUG_TYPE "arm-pseudo"