Match declaration to definition (missed a few).

llvm-svn: 75456
diff --git a/llvm/lib/CodeGen/MachOWriter.h b/llvm/lib/CodeGen/MachOWriter.h
index e8e57934..e5c7ee6 100644
--- a/llvm/lib/CodeGen/MachOWriter.h
+++ b/llvm/lib/CodeGen/MachOWriter.h
@@ -24,9 +24,9 @@
   class Mangler;
   class MachineRelocation;
   class MachOCodeEmitter;
-  class MachODySymTab;
-  class MachOHeader;
-  class MachOSection;
+  struct MachODySymTab;
+  struct MachOHeader;
+  struct MachOSection;
   struct MachOSym;
   class TargetData;
   class TargetMachine;