fix clang build

Summary: PipelineBase needs something virtual so we can dynamic_cast from it down to the real pipeline, as we do in Proxy

Test Plan: built proxy with clang

Reviewed By: hans@fb.com

Subscribers: fugalh, mathieubaudet, folly-diffs@, jsedgwick, yfeldblum, chalfant

FB internal diff: D2075855

Tasks: 7077419, 7077417

Signature: t1:2075855:1431708780:03ce8d8f40cdb56d24baf75f4dc29004e5ea4c7a

folly/wangle/channel/Pipeline.h

@@ -33,6 +33,8 @@ class PipelineManager {
 
 class PipelineBase {
  public:
+  virtual ~PipelineBase() {}
+
   void setPipelineManager(PipelineManager* manager) {
     manager_ = manager;
   }