examples: predicate ping example on p.SO_BINDTODEVICE.

* examples/ping.lua: If either p.SOCK_RAW or p.SO_BINDTODEVICE
are not defined, then the underlying OS libraries didn't have
the necessary functionality for luaposix to bind to... so skip
the example in that case.

Signed-off-by: Gary V. Vaughan <gary@gnu.org>