Fixed unsigned warning

src/google/protobuf/arena_impl.h

diff --git a/src/google/protobuf/arena_impl.h b/src/google/protobuf/arena_impl.h
index a0fb106..b727ca7 100644
--- a/src/google/protobuf/arena_impl.h
+++ b/src/google/protobuf/arena_impl.h
@@ -52,7 +52,7 @@
 
 inline size_t AlignUpTo8(size_t n) {
   // Align n to next multiple of 8 (from Hacker's Delight, Chapter 3.)
-  return (n + 7) & -8;
+  return (n + 7) & (size_t)-8;
 }
 
 // This class provides the core Arena memory allocation library. Different
@@ -306,8 +306,8 @@
  public:
   // kBlockHeaderSize is sizeof(Block), aligned up to the nearest multiple of 8
   // to protect the invariant that pos is always at a multiple of 8.
-  static const size_t kBlockHeaderSize = (sizeof(Block) + 7) & -8;
-  static const size_t kSerialArenaSize = (sizeof(SerialArena) + 7) & -8;
+  static const size_t kBlockHeaderSize = (sizeof(Block) + 7) & (size_t)-8;
+  static const size_t kSerialArenaSize = (sizeof(SerialArena) + 7) & (size_t)-8;
   static_assert(kBlockHeaderSize % 8 == 0,
                 "kBlockHeaderSize must be a multiple of 8.");
   static_assert(kSerialArenaSize % 8 == 0,