core: fix __libc_free return type signature
core: fix __libc_free return type signature

The alias for free() function should return void instead of void*.
This mismatched return type causes compilation error on gcc 8.1.1-5.

Message-Id: <3f508d9dbb2f2a1c03b9aa134f6e8e31115288d8@scylladb.com>

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