memory: add [[gnu::malloc]] attribute to memalign()
memory: add [[gnu::malloc]] attribute to memalign()

Fedora 31's gcc complains that the attributes on
__libc_memalign() don't match memalign()'s, so humor it.

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