Fix declaration of c-api function wasmtime_component_linker_instance_add_func

[bartbes]

I was trying to use the C api and could not figure out how to implement a finalizer that takes no arguments. Turns out, the c declaration simply did not match the rust implementation.

See the rust specification of finalizer here:

wasmtime/crates/c-api/src/component/linker.rs

Lines 105 to 112 in d2f5118

	pub unsafe extern "C" fn wasmtime_component_linker_instance_add_func(
	linker_instance: &mut wasmtime_component_linker_instance_t,
	name: *const u8,
	name_len: usize,
	callback: wasmtime_component_func_callback_t,
	data: *mut c_void,
	finalizer: Option<extern "C" fn(*mut c_void)>,
	) -> Option<Box<wasmtime_error_t>> {

extern "C" fn(*mut c_void)> of course matches void(*)(void*), not void(*)().

[alexcrichton]

Thanks!