Add a missing forward declaration.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@40896 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/lib/CodeGen/VirtRegMap.h b/lib/CodeGen/VirtRegMap.h
index e5d4b22..da20d4a 100644
--- a/lib/CodeGen/VirtRegMap.h
+++ b/lib/CodeGen/VirtRegMap.h
@@ -25,6 +25,7 @@
 
 namespace llvm {
   class MachineInstr;
+  class MachineFunction;
   class TargetInstrInfo;
 
   class VirtRegMap {