message: fix the build on OSX
diff: // Copyright (c) 2018-present The Alive2 Authors.
  // Distributed under the MIT license that can be found in the LICENSE file.
  +#include "cache/cache.h"
  #include "ir/type.h"
  #include "llvm_util/llvm2alive.h"
  #include "smt/expr.h"
  @@ -65,6 +66,7 @@ std::unique_ptr<llvm::Module> openInputFile(llvm::LLVMContext &Context,
  }
  optional<smt::smt_initializer> smt_init;
  +unique_ptr<Cache> cache;
  void execFunction(llvm::Function &F, llvm::TargetLibraryInfoWrapperPass &TLI,
  unsigned &successCount, unsigned &errorCount) {
  