Mark miniDump() as pure virtual.

Fixes a build error when building with -O0 due to missing vtable entry.

Test: make NATIVE_COVERAGE=true
COVERAGE_PATHS=frameworks/native/services/surfaceflinger

Change-Id: Iaefb6921b4fe95a2c138444a0cfba2ecb739550f
diff --git a/services/surfaceflinger/TimeStats/TimeStats.h b/services/surfaceflinger/TimeStats/TimeStats.h
index 4e040a3..2bcb568 100644
--- a/services/surfaceflinger/TimeStats/TimeStats.h
+++ b/services/surfaceflinger/TimeStats/TimeStats.h
@@ -41,7 +41,7 @@
 
     virtual void parseArgs(bool asProto, const Vector<String16>& args, std::string& result) = 0;
     virtual bool isEnabled() = 0;
-    virtual std::string miniDump();
+    virtual std::string miniDump() = 0;
 
     virtual void incrementTotalFrames() = 0;
     virtual void incrementMissedFrames() = 0;