Formatting change; NFC

I'm adding to the list of forward declares in a PR for #embed, so this
won't be a formatting-only change for long.

Author: AaronBallman

clang/include/clang/Lex/PPCallbacks.h

@@ -22,11 +22,11 @@
 #include "llvm/ADT/StringRef.h"
 
 namespace clang {
-  class Token;
-  class IdentifierInfo;
-  class MacroDefinition;
-  class MacroDirective;
-  class MacroArgs;
+class Token;
+class IdentifierInfo;
+class MacroDefinition;
+class MacroDirective;
+class MacroArgs;
 
 /// This interface provides a way to observe the actions of the
 /// preprocessor as it does its thing.