kconfig: remove unnecessary cast in sym_get_string()

The explicit casting from (char *) to (const char *) is unneeded.

Signed-off-by: Masahiro Yamada <[e-mail]>

scripts/kconfig/symbol.c

···
 	default:
 		;
 	}
-	return (const char *)sym->curr.val;
+	return sym->curr.val;
 }
 
 bool sym_is_changeable(const struct symbol *sym)