folly/io/async/ssl/OpenSSLTransportCertificate.h: mark overriding functions with "override"

Summary:
This avoids the following errors:

  folly/io/async/ssl/OpenSSLTransportCertificate.h:31:11: error: '~OpenSSLTransportCertificate' overrides a destructor but is not marked 'override' [-Werror,-Wsuggest-destructor-override]

Reviewed By: r-barnes

Differential Revision: D33397487

fbshipit-source-id: 4237f5ad36166189b0bbd28dfcf268e0ef4b40f1

folly/io/async/ssl/OpenSSLTransportCertificate.h

@@ -28,7 +28,7 @@ namespace folly {
  */
 class OpenSSLTransportCertificate : public AsyncTransportCertificate {
  public:
-  virtual ~OpenSSLTransportCertificate() = default;
+  virtual ~OpenSSLTransportCertificate() override = default;
 
   /**
    * Returns an X509 structure associated with this Certificate. This may be