Prune some includes and forward declarations.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@153415 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/lib/Target/ARM/ARMELFWriterInfo.h b/lib/Target/ARM/ARMELFWriterInfo.h
index 1c4e532..6a84f8a 100644
--- a/lib/Target/ARM/ARMELFWriterInfo.h
+++ b/lib/Target/ARM/ARMELFWriterInfo.h
@@ -17,6 +17,7 @@
 #include "llvm/Target/TargetELFWriterInfo.h"
 
 namespace llvm {
+  class TargetMachine;
 
   class ARMELFWriterInfo : public TargetELFWriterInfo {
   public: