libstdc++: Fix -Wunused-parameter in <experimental/internet>

libstdc++-v3/ChangeLog:

	* include/experimental/internet (address_v4::to_string): Remove
	unused parameter name.

(cherry picked from commit 8ee74c5a)

libstdc++-v3/include/experimental/internet

@@ -252,7 +252,7 @@ namespace ip
       __string_with<_Allocator>
       to_string(const _Allocator& __a = _Allocator()) const
       {
-	auto __write = [__addr = to_uint()](char* __p, size_t __n) {
+	auto __write = [__addr = to_uint()](char* __p, size_t) {
 	  auto __to_chars = [](char* __p, uint8_t __v) {
 	    unsigned __n = __v >= 100u ? 3 : __v >= 10u ? 2 : 1;
 	    std::__detail::__to_chars_10_impl(__p, __n, __v);