Fix compiler warnings from clang 5.0

[ 43%] Building CXX object c++/src/CMakeFiles/orc.dir/io/InputStream.cc.o

In file included from /home/travis/build/apache/orc/c++/src/io/InputStream.cc:20:

/home/travis/build/apache/orc/c++/src/io/InputStream.hh:75:13: error: '~SeekableArrayInputStream' overrides a destructor but is not marked 'override' [-Werror,-Winconsistent-missing-destructor-override]

    virtual ~SeekableArrayInputStream();

            ^

/home/travis/build/apache/orc/c++/src/io/InputStream.hh:53:13: note: overridden virtual function is here

    virtual ~SeekableInputStream();

            ^

/home/travis/build/apache/orc/c++/src/io/InputStream.hh:104:13: error: '~SeekableFileInputStream' overrides a destructor but is not marked 'override' [-Werror,-Winconsistent-missing-destructor-override]

    virtual ~SeekableFileInputStream();

            ^

/home/travis/build/apache/orc/c++/src/io/InputStream.hh:53:13: note: overridden virtual function is here

    virtual ~SeekableInputStream();

            ^