[Serialization] Add an extra include.

It seems to be getting picked up from one of the other headers in master,
but not in 602 (locally), so it seems correct to add here.

No functionality change.

Swift SVN r25230

include/swift/Serialization/ModuleFile.h

@@ -28,6 +28,7 @@
 #include "llvm/ADT/Fixnum.h"
 #include "llvm/ADT/TinyPtrVector.h"
 #include "llvm/Bitcode/BitstreamReader.h"
+#include "llvm/Support/MemoryBuffer.h"
 
 namespace llvm {
   class BitstreamCursor;