added <array> header

[MatyushinMA]

When trying to build the lib on ubuntu with cmake using clang++-11 with libc++, the following error occurs:

/home/lib/pytorch/third_party/gloo/gloo/transport/tcp/device.cc:152:39: error: implicit instantiation of undefined template 'std::__1::array<char, 64>' std::array<char, HOST_NAME_MAX> hostname; ^ /usr/lib/llvm-10/bin/../include/c++/v1/__tuple:219:64: note: template is declared here template <class _Tp, size_t _Size> struct _LIBCPP_TEMPLATE_VIS array;

which is because the header is absent in the gloo/transport/tcp/device.cc

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