Fix build with current fmt (master branch)

[phprus]

Build error:

In file included from /..../unordered_dense/test/app/counter.cpp:1:
/..../unordered_dense/test/app/counter.h: In function 'bool operator==(const counter::data_t&, const counter::data_t&)':
/..../unordered_dense/test/app/counter.h:31:30: error: 'memcmp' is not a member of 'std'; did you mean 'wmemcmp'?
   31 |             return 0 == std::memcmp(&a, &b, sizeof(data_t));
      |                              ^~~~~~

[martinus]

Thanks!