config/Block: make "used" mutable

src/config/Block.hxx

@@ -69,7 +69,7 @@ struct ConfigBlock {
 	 * This flag is false when nobody has queried the value of
 	 * this option yet.
 	 */
-	bool used = false;
+	mutable bool used = false;
 
 	explicit ConfigBlock(int _line=-1)
 		:line(_line) {}

src/config/Global.cxx

@@ -83,7 +83,7 @@ config_get_param(ConfigOption option) noexcept
 const ConfigBlock *
 config_get_block(ConfigBlockOption option) noexcept
 {
-	ConfigBlock *block = config_data.blocks[unsigned(option)];
+	const auto *block = config_data.blocks[unsigned(option)];
 	if (block != nullptr)
 		block->used = true;
 	return block;