This fixes the build with gcc 14:

contrib/kyua/engine/prepare/prepare_all.cpp:56:16: error: declaration of 'handler' shadows a member of 'engine::prepare::prepare_all' [-Werror=shadow]

56 |     for (auto& handler : prepare::handlers()) {
   |                ^~~~~~~

In file included from /home/siva/f/main/contrib/kyua/engine/prepare/prepare_all.hpp:35,

from /home/siva/f/main/contrib/kyua/engine/prepare/prepare_all.cpp:29:

/home/siva/f/main/contrib/kyua/engine/prepare/prepare.hpp:51:15: note: shadowed declaration is here

51 | class handler {
   |               ^

Fixes: edb230c4af499203d7a6894b3711fe6574b26040