Fix Windows linker errors in skia_opts[_ssse3]

These are errors like:

    skia_opts.lib(skia_opts.skblitrow_opts_sse2.obj) : error LNK2001: unresolved external symbol "public: static int SkRefCnt::SkInstanceCountHelper::gInstanceCount" (?gInstanceCount@SkInstanceCountHelper@SkRefCnt@@2HA)