[AVR] Add missing #includes

A previous change seems to have remove #includes from header files. This
fixes the build.

llvm-svn: 294427
diff --git a/llvm/lib/Target/AVR/MCTargetDesc/AVRMCCodeEmitter.cpp b/llvm/lib/Target/AVR/MCTargetDesc/AVRMCCodeEmitter.cpp
index e6dc886..c3d43eb 100644
--- a/llvm/lib/Target/AVR/MCTargetDesc/AVRMCCodeEmitter.cpp
+++ b/llvm/lib/Target/AVR/MCTargetDesc/AVRMCCodeEmitter.cpp
@@ -25,6 +25,7 @@
 #include "llvm/MC/MCInstrInfo.h"
 #include "llvm/MC/MCRegisterInfo.h"
 #include "llvm/MC/MCSubtargetInfo.h"
+#include "llvm/Support/Casting.h"
 #include "llvm/Support/raw_ostream.h"
 
 #define DEBUG_TYPE "mccodeemitter"