add suggested override

Using Chromium's clang with skia_use_metal=true produces

    ../src/gpu/mtl/GrMtlCommandBuffer.h:25:5: warning: '~GrMtlCommandBuffer' overrides a destructor but is not marked 'override' [-Wsuggest-destructor-override]
        ~GrMtlCommandBuffer();

Change-Id: I177a6645a37cf9853b9284798eb48997428e1854
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/313808
Commit-Queue: Mike Klein <mtklein@google.com>
Commit-Queue: Brian Salomon <bsalomon@google.com>
Auto-Submit: Mike Klein <mtklein@google.com>
Reviewed-by: Brian Salomon <bsalomon@google.com>

src/gpu/mtl/GrMtlCommandBuffer.h

@@ -22,7 +22,7 @@ class GrMtlOpsRenderPass;
 class GrMtlCommandBuffer : public SkRefCnt {
 public:
     static sk_sp<GrMtlCommandBuffer> Make(id<MTLCommandQueue> queue);
-    ~GrMtlCommandBuffer();
+    ~GrMtlCommandBuffer() override;
 
     bool commit(bool waitUntilCompleted);
     bool hasWork() { return fHasWork; }