[c++26 fix] include <exception> in Gen.hpp

[marcia-pedals]

When compiling in a c++26 project, I get this error:

  rapidcheck/Gen.hpp:72:22: error: calling 'current_exception' with incomplete return type 'exception_ptr'
     72 |     auto exception = std::current_exception();
        |                      ^~~~~~~~~~~~~~~~~~~~~~~~
  .../exception/operations.h:36:33: note: forward declaration of 'std::exception_ptr'
     36 | class _LIBCPP_EXPORTED_FROM_ABI exception_ptr;
        |                                 ^

Including <exception> fixes it, and this seems correct to me because https://en.cppreference.com/w/cpp/error/exception_ptr.html says that it's defined in <exception>.