selfhost: graduate SK_* defs to skind enum

selfhost/cmd/wcc/cgenexpr.ww

@@ -861,7 +861,7 @@ fn cgdot(c: *cgen, n: *node) void = {
 		};
 	};
 	// Module-qualified value reference: `mod.name` where `mod`
-	// is N_IDENT bound as SK_USE and the leaf isn't a local.
+	// is N_IDENT bound as skind.SK_USE and the leaf isn't a local.
 	// Treat as a SB symbol — `MOVQ leaf(SB), AX`. Same fallback
 	// the C cgen takes when bt is NULL/tyerr.
 	if (lhs != nil) {