cgen: fix building with `-no-preludes -no-builtin` (part 1)

vlib/v/gen/c/cgen.v

@@ -7203,6 +7203,11 @@ fn (mut g Gen) interface_table() string {
 		if isym.kind != .interface_ {
 			continue
 		}
+		if isym.info !is ast.Interface {
+			// Do not remove this check, `isym.info` could be `&IError`.
+			// dump(isym)
+			continue
+		}
 		inter_info := isym.info as ast.Interface
 		if inter_info.is_generic {
 			continue