libstdc++: Fix up all_pedantic_errors.cc

libstdc++: Fix up all_pedantic_errors.cc

On x86_64-linux I see
On x86_64-linux
FAIL: 17_intro/headers/c++1998/all_pedantic_errors.cc  -std=gnu++17 (test for excess errors)
Excess errors:
/home/jakub/src/gcc/obj20/x86_64-pc-linux-gnu/libstdc++-v3/include/bits/random.tcc:3681: error: ISO C++ does not support '__int128' for 'type name' [-Wpedantic]
/home/jakub/src/gcc/obj20/x86_64-pc-linux-gnu/libstdc++-v3/include/bits/random.tcc:3698: error: ISO C++ does not support '__int128' for 'type name' [-Wpedantic]
This fixes it by adding __extension__.

2025-12-17  Jakub Jelinek  <jakub@redhat.com>

	* include/bits/random.tcc (std::generate_canonical): Use
	__extension__ before __generate_canonical_{pow2,any} calls with
	unsigned __int128 template arguments.