lang/gforth: try to fix another warning-becomes-error with GCC 14

Old gforth_callback_ffcall() signature looks dubious and does not
match the API of `devel/libffcall' [-Wincompatible-pointer-types]:

callback.h:58:35: note: expected 'callback_function_t' {aka

'void (*)(void *, struct vacall_alist *)'} but argument is of

type 'void (*)(void ***, void *)'

PR: 281238