tests: settings: file_littlefs: move prototype to header file

Move `config_setup_littlefs()` prototype to header file, so that potential
type mismatch will be detected by both `settings_setup_littlefs.c`
implementing it and `settings_test_littlefs.c` using it.

Signed-off-by: Marcin Niestroj <m.niestroj@emb.dev>