Fix compilation errors on Windows

MSVC doesn't support fSpinLock=ATOMIC_FLAG_INIT; in the class definition, nor in the class constructor initializer list

include/cling/Interpreter/Transaction.h

@@ -17,6 +17,7 @@
 
 #include "llvm/ADT/SmallVector.h"
 #include "llvm/ADT/StringRef.h"
+#include "llvm/IR/Module.h"
 
 #include <memory>
 
@@ -34,7 +35,6 @@ namespace clang {
 
 namespace llvm {
   class raw_ostream;
-  class Module;
 }
 
 namespace cling {