Add missing override keyword

Fuchsia builds with Clang, and it's a bit pickier about always using
override when it's appropriate.

Test: None

libunwindstack/include/unwindstack/Memory.h

@@ -101,7 +101,7 @@ class MemoryFileAtOffset : public Memory {
 
   size_t Size() { return size_; }
 
-  void Clear();
+  void Clear() override;
 
  protected:
   size_t size_ = 0;