Add #include (introduced by dependence on DeclGroup)

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@57274 91177308-0d34-0410-b5e6-96231b3b80d8

lib/Rewrite/Rewriter.cpp

@@ -14,6 +14,7 @@
 
 #include "clang/Rewrite/Rewriter.h"
 #include "clang/AST/Stmt.h"
+#include "clang/AST/Decl.h"
 #include "clang/Lex/Lexer.h"
 #include "clang/Basic/SourceManager.h"
 #include "llvm/Support/raw_ostream.h"