fix header inclusion

Author: scotthart

google/cloud/internal/http_header.h

@@ -16,13 +16,13 @@
 #define GOOGLE_CLOUD_CPP_GOOGLE_CLOUD_INTERNAL_HTTP_HEADER_H
 
 #include "google/cloud/version.h"
-#if defined(_WIN64) || defined(__LP64__) || defined(__x86_64__) || \
-    defined(__ppc64__)
+#if UINTPTR_MAX == UINT64_MAX
 #include "absl/container/flat_hash_map.h"
 #else
 #include <unordered_map>
 #endif
 #include "absl/strings/ascii.h"
+#include <cstdint>
 #include <string>
 #include <vector>