cgen: fix illegal character encoding with rune consts above 127 (#11550)

vlib/v/gen/c/cgen.v

@@ -5210,7 +5210,7 @@ fn (mut g Gen) const_decl_precomputed(mod string, name string, ct_value ast.Comp
 		}
 		rune {
 			rune_code := u32(ct_value)
-			if rune_code <= 255 {
+			if rune_code <= 127 {
 				if rune_code in [`"`, `\\`, `'`] {
 					return false
 				}

vlib/v/tests/high_ascii_const_rune_test.v

@@ -0,0 +1,5 @@
+const accented = `á`
+
+fn test_high_ascii_const() {
+	assert u32(accented) == 225
+}