Fix declaration of the ArgsFlags concept.

[bangerth]

The syntax used so far triggers a compiler warning for me:

/home/bangerth/tmp/g/modularizer/importizer/src/OptProcessor.cc:34:3: warning: testing if a concept-id is a valid expression; add ‘requires’ to check satisfaction [-Wmissing-requires]
   34 |   std::derived_from<T, args::FlagBase>;
      |   ^
      |   requires 

This is because the syntax

requires (T flags) {
  std::derived_from<T, args::FlagBase>;
};

only checks that the enclosed expression std::derived_from<T, args::FlagBase> is valid, not that it is true. This patch fixes it to what I assume was originally intended.