Prune some includes and forward declarations.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@153429 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/lib/MC/MCAsmBackend.cpp b/lib/MC/MCAsmBackend.cpp
index 51c3977..0b2e4ae 100644
--- a/lib/MC/MCAsmBackend.cpp
+++ b/lib/MC/MCAsmBackend.cpp
@@ -8,6 +8,7 @@
 //===----------------------------------------------------------------------===//
 
 #include "llvm/MC/MCAsmBackend.h"
+#include "llvm/MC/MCFixupKindInfo.h"
 using namespace llvm;
 
 MCAsmBackend::MCAsmBackend()