Silence -Wdeprecated warning on clang

* definition of implicit copy constructor for 'SnappySinkAllocator'
  is deprecated because it has a user-declared destructor.

snappy.cc

@@ -2341,7 +2341,6 @@ bool SnappyScatteredWriter<Allocator>::SlowAppendFromSelf(size_t offset,
 class SnappySinkAllocator {
  public:
   explicit SnappySinkAllocator(Sink* dest) : dest_(dest) {}
-  ~SnappySinkAllocator() {}
 
   char* Allocate(int size) {
     Datablock block(new char[size], size);