Minor compilation warning

[Lecrapouille]

Always nice to compile with -Wshadow to catch some silent error (not the case here):

FileDialogs.hpp:1768:25: warning: declaration of ‘flag’ shadows a member of ‘pfd::message’ [-Wshadow]
 1768 |             std::string flag = "--";
      |                         ^~~~
In file included from XXX.cpp:
FileDialogs.hpp:138:16: note: shadowed declaration is here
  138 |     enum class flag
      |                ^~~~