Mark overridden destructors with 'override' and remove 'virtual'

This silences a new warning in clang 5.0

Change-Id: Ieb5b75a6ffed60107c3fd16075d2ecfd515b55e8
Reviewed-on: https://skia-review.googlesource.com/10006
Commit-Queue: Brian Salomon <bsalomon@google.com>
Reviewed-by: Mike Klein <mtklein@chromium.org>
diff --git a/bench/BlurRectBench.cpp b/bench/BlurRectBench.cpp
index 774bafa..1631302 100644
--- a/bench/BlurRectBench.cpp
+++ b/bench/BlurRectBench.cpp
@@ -105,7 +105,7 @@
 public:
     BlurRectSeparableBench(SkScalar rad) : INHERITED(rad) { }
 
-    ~BlurRectSeparableBench() {
+    ~BlurRectSeparableBench() override {
         SkMask::FreeImage(fSrcMask.fImage);
     }