Prune RelocVisitor.h include to avoid including COFF.h from MCJIT.h

This helps to mitigate the conflict between COFF.h and winnt.h, which is
PR28399.

llvm-svn: 274637
diff --git a/llvm/lib/ExecutionEngine/RuntimeDyld/RuntimeDyldELF.h b/llvm/lib/ExecutionEngine/RuntimeDyld/RuntimeDyldELF.h
index cca80af..82931b9 100644
--- a/llvm/lib/ExecutionEngine/RuntimeDyld/RuntimeDyldELF.h
+++ b/llvm/lib/ExecutionEngine/RuntimeDyld/RuntimeDyldELF.h
@@ -20,6 +20,9 @@
 using namespace llvm;
 
 namespace llvm {
+namespace object {
+class ELFObjectFileBase;
+}
 
 class RuntimeDyldELF : public RuntimeDyldImpl {