Bug 23068: (RM follow-up) Make Net::Netmask required

Bug 23068: (RM follow-up) Make Net::Netmask required

Net::Netmask is used in the new Plack::Middleware::RealIP module, which
is in turn used in the psgi plack startup script.  We don't have access
to C4::Context inside the startup script and as such don't have access
to the config to work out if loading this module should be optional.

As such, I'm taking the easy route and just making the dependancy a
required one. It's small and already packaged for debian and as such
should not cause anyone any issues.

Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>