Include <functional>

This is necessary to avoid compilation errors using recent libc++
versions with C++23 due to libc++ taking steps to remove transitive
includes in newer C++ versions [0].

For a more concrete example, this program builds using Clang 16.0.0 and
C++20, but fails to build with C++23:

    #include <magic_enum.hpp>
    enum class ec { RED };
    template <>
    constexpr auto magic_enum::customize::enum_name(ec val) noexcept
        -> magic_enum::customize::customize_t {
      switch (val) {
      case ec::RED: return "Red";
      };
      return invalid_tag;
    }

The compiler hits the "too many errors" threshold when building with
C++23, but I believe this is due to compiling failing to recover after
the first error. The error message starts with:

    include % /usr/local/opt/llvm/bin/clang++ -std=c++2b -c test.cpp
    In file included from test.cpp:1:
    ./magic_enum.hpp:324:61: error: no member named 'equal_to' in namespace 'std'
      return std::is_same_v<std::decay_t<BinaryPredicate>, std::equal_to<string_view::value_type>> ||
                                                           ~~~~~^
    ./magic_enum.hpp:324:93: error: expected '(' for function-style cast or type construction
      return std::is_same_v<std::decay_t<BinaryPredicate>, std::equal_to<string_view::value_type>> ||
                                                                         ~~~~~~~~~~~~~~~~~~~~~~~^
    ./magic_enum.hpp:324:96: error: expected expression
      return std::is_same_v<std::decay_t<BinaryPredicate>, std::equal_to<string_view::value_type>> ||
                                                                                                   ^

std::equal_to is defined in <functional>, but <functional> is not
directly included by magic_enum.hpp. <string> [1] and <string_view> [2]
both include <functional> for C++20 and earlier, but they no longer
include <functional> in C++23 and later, so a definition for
std::equal_to is no longer visible, leading to the above error.

Directly including <functional> fixes this error, but I'm not sure there
aren't similar errors.

[0]: llvm/llvm-project@8ff2d6a

[1]: https://github.com/llvm/llvm-project/blob/llvmorg-16.0.0/libcxx/include/string#L4625

[2]: https://github.com/llvm/llvm-project/blob/llvmorg-16.0.0/libcxx/include/string_view#L1027