PARQUET-561: Add destructor to PIMPL

Without this destuctor, the SerializedFile class cannot be destructed and leaks memory.

Author: Uwe L. Korn <uwelk@xhochy.com>

Closes apache#77 from xhochy/destructor-for-pimpl and squashes the following commits:

0f80966 [Uwe L. Korn] Add destructor to PIMPL

cpp/src/parquet/file/reader.h

@@ -69,6 +69,7 @@ class ParquetFileReader {
  public:
   // Forward declare the PIMPL
   struct Contents {
+    virtual ~Contents() {}
     // Perform any cleanup associated with the file contents
     virtual void Close() = 0;