Add virtual destructors to IReader and IWriter

src/butil/reader_writer.h

@@ -26,6 +26,8 @@ namespace butil {
 // The simplest implementation is to embed a file descriptor and read from it.
 class IReader {
 public:
+    virtual ~IReader() {}
+
     // Semantics of parameters and return value are same as readv(2) except that
     // there's no `fd'.
     virtual ssize_t ReadV(const iovec* iov, int iovcnt) = 0;
@@ -35,6 +37,8 @@ public:
 // The simplest implementation is to embed a file descriptor and writev into it.
 class IWriter {
 public:
+    virtual ~IWriter() {}
+
     // Semantics of parameters and return value are same as writev(2) except that
     // there's no `fd'.
     // WriteV is required to submit data gathered by multiple appends in one