Add missing forward declaration.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@79481 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/lib/CodeGen/MachOWriter.h b/lib/CodeGen/MachOWriter.h
index e5c7ee6..9d8e9d4 100644
--- a/lib/CodeGen/MachOWriter.h
+++ b/lib/CodeGen/MachOWriter.h
@@ -22,6 +22,7 @@
   class Constant;
   class GlobalVariable;
   class Mangler;
+  class MachineBasicBlock;
   class MachineRelocation;
   class MachOCodeEmitter;
   struct MachODySymTab;