[Debuginfod][unittest] Include llvm/Support/StringExtras.h

[Caslyn]

Summary

Include the "llvm/ADT/StringExtras.h" in unittest to resolve a
"use of undeclared identifier 'utostr'" error that surfaces on mac-x64
builds after https://reviews.llvm.org/D155178.

[haowei]

This test is breaking on Windows as well.

[phosek]

LGTM

llvm/unittests/Debuginfod/HTTPServerTests.cpp

 //===-- llvm/unittest/Support/HTTPServer.cpp - unit tests -------*- C++ -*-===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 //===----------------------------------------------------------------------===//
 
+#include "llvm/ADT/StringExtras.h"
 #include "llvm/Debuginfod/HTTPClient.h"
 #include "llvm/Debuginfod/HTTPServer.h"
 #include "llvm/Support/Error.h"
 #include "llvm/Support/ThreadPool.h"
 #include "llvm/Testing/Support/Error.h"
 #include "gmock/gmock.h"
 #include "gtest/gtest.h"