YJIT: Correct signature of rb_yjit_root_mark()

Even though unused, it's supposed to take a pointer like the C side
expects.

yjit/src/invariants.rs

@@ -348,7 +348,7 @@ pub extern "C" fn rb_yjit_constant_state_changed(id: ID) {
 /// Callback for marking GC objects inside [Invariants].
 /// See `struct yjijt_root_struct` in C.
 #[no_mangle]
-pub extern "C" fn rb_yjit_root_mark() {
+pub extern "C" fn rb_yjit_root_mark(_: *mut c_void) {
     // Call rb_gc_mark on exit location's raw_samples to
     // wrap frames in a GC allocated object. This needs to be called
     // at the same time as root mark.