Add missing forward declaration.

This should fix our modules builds.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@306332 91177308-0d34-0410-b5e6-96231b3b80d8

include/llvm/MC/MCWinCOFFObjectWriter.h

@@ -13,6 +13,7 @@
 namespace llvm {
 
 class MCAsmBackend;
+class MCContext;
 class MCFixup;
 class MCObjectWriter;
 class MCValue;