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>
1 file changed