Add missing =0 on interface

Change-Id: Ife0cf573862ba527246a9dbcdda8e03460cf074e
Test: builds
Flag: EXEMPT build fix

Author: jreck

services/surfaceflinger/Scheduler/EventThread.h

@@ -144,7 +144,7 @@ class EventThread {
 
     // An elevated number of queued buffers in the server is detected. This propagates a
     // flag to Choreographer indicating that buffer stuffing recovery should begin.
-    virtual void addBufferStuffedUids(BufferStuffingMap bufferStuffedUids);
+    virtual void addBufferStuffedUids(BufferStuffingMap bufferStuffedUids) = 0;
 };
 
 struct IEventThreadCallback {