foreign_ptr: declare destroy as a static method
foreign_ptr: declare destroy as a static method

As noted by Nadav Har'El <nyh@scylladb.com>:
> If this function doesn't use "this", it should be declared static!

Signed-off-by: Benny Halevy <23c997c90eb8537635e9392d48e7af1bf1b2ca22@scylladb.com>
Message-Id: <20211128121919.173440-1-23c997c90eb8537635e9392d48e7af1bf1b2ca22@scylladb.com>

avikivity/seastar,avikivity/seastar,syuu1228/seastar,scylladb/seastar,scylladb/seastar,syuu1228/seastar,scylladb/seastar,avikivity/seastar,syuu1228/seastar