memory: conditionally add [[gnu::alloc_size]] attribute to memalign() for glibc 2.30
memory: conditionally add [[gnu::alloc_size]] attribute to memalign() for glibc 2.30

glibc 2.30 added the alloc_size attribute to memalign(), so we have to follow
suit, or the compiler complains about mismatched attributes between glibc's
declarations and our definitions.

syuu1228/seastar,cloudius-systems/seastar,scylladb/seastar,dreamsxin/seastar,avikivity/seastar,avikivity/seastar,syuu1228/seastar,scylladb/seastar,dreamsxin/seastar,cloudius-systems/seastar,scylladb/seastar,syuu1228/seastar,cloudius-systems/seastar,dreamsxin/seastar,avikivity/seastar