Make vctrs_init_altrep_lazy_character function signatures consistent

[georgestagg]

In src/altrep-lazy-character.c we have,

void vctrs_init_altrep_lazy_character(DllInfo* dll) {
[...]
}

but in src/init.c we have,

extern r_obj* vctrs_init_altrep_lazy_character(DllInfo*);

This breaks vctrs 0.6.3 under webR because Wasm requires consistent function signatures across compilation units. The function doesn't seem to return anything, so I've switched the extern definition to use a return type of void.

[DavisVaughan]

Thanks!