lxc.functions isn't a shell script

lxc.functions.in is meant to be sourced, not to be called as a script.
So as it's not executable and not meant to be, it shouldn't have
a /bin/sh shebang.

This fixes an error reported by lintian.

Signed-off-by: Stéphane Graber <stgraber@ubuntu.com>
Acked-by: Serge E. Hallyn <serge.hallyn@ubuntu.com>