osmo_io: Fix misaligment of iofd->cmsg used as struct cmsghdr
Fixes following ASAN runtime error: """ src/stream.c:398:47: runtime error: member access within misaligned address 0x516000012b81 for type 'struct cmsghdr', which requires 8 byte alignment 0x516000012b81: note: pointer points here 51 00 00 00 30 00 00 00 00 00 00 00 84 00 00 00 01 00 00 00 00 00 00 00 00 00 00 00 00 00 00 03 ^ """
Current GCC documentation [1] states that __attribute__ ((__aligned__ (alignment))) is the older sytnax. The newer _Alignas mimics the standarized alignas() in C23. Since we build with CLAGS=-std=gnu11 we are fine using _Alignas, which was introduced in C11.