Fix build with libc++

[zifeitong]

std::experimental::simd in libc++ is incomplete:

In file included from <source>:1:
/app/raw.githubusercontent.com/qlibs/mph/main/mph:1144:26: error: no member named 'any_of' in namespace 'std::experimental'
 1144 |       std::experimental::any_of(match),
      |                          ^~~~~~
/app/raw.githubusercontent.com/qlibs/mph/main/mph:1145:53: error: no member named 'find_first_set' in namespace 'std::experimental'
 1145 |       result_type(values[index + std::experimental::find_first_set(match)]),
      |                                                     ^~~~~~~~~~~~~~
/app/raw.githubusercontent.com/qlibs/mph/main/mph:1145:75: error: expected ']'
 1145 |       result_type(values[index + std::experimental::find_first_set(match)]),
      |                                                                           ^
/app/raw.githubusercontent.com/qlibs/mph/main/mph:1145:25: note: to match this '['
 1145 |       result_type(values[index + std::experimental::find_first_set(match)]),
      |                         ^

Tests:

• lib++: https://godbolt.org/z/sodGhYG3q
• libstdc++: https://godbolt.org/z/3zjYdd4vn