RustWrapper: add missing include

This is required after LLVM change 3c4410dfcaaf (aka
https://reviews.llvm.org/D121168) did some includes cleanup.

compiler/rustc_llvm/llvm-wrapper/RustWrapper.cpp

@@ -9,6 +9,7 @@
 #include "llvm/Object/Archive.h"
 #include "llvm/Object/COFFImportFile.h"
 #include "llvm/Object/ObjectFile.h"
+#include "llvm/Pass.h"
 #include "llvm/Bitcode/BitcodeWriterPass.h"
 #include "llvm/Support/Signals.h"
 #include "llvm/ADT/Optional.h"