Add install Net::SDP

[jshawl]

Without installing, users receive the following error:

Can't locate Net/SDP.pm in @INC (you may need to install the Net::SDP
module) (@INC contains: /Library/Perl/5.18/darwin-thread-multi-2level
/Library/Perl/5.18 /Network/Library/Perl/5.18/darwin-thread-multi-2level
/Network/Library/Perl/5.18 /Library/Perl/Updates/5.18.2
/System/Library/Perl/5.18/darwin-thread-multi-2level
/System/Library/Perl/5.18
/System/Library/Perl/Extras/5.18/darwin-thread-multi-2level
/System/Library/Perl/Extras/5.18 .) at shairport.pl line 48.
BEGIN failed--compilation aborted at shairport.pl line 48.

[abrasive]

This patch is against the originally released Perl/C hybrid design (versions 0.0x). The current master branch is a monolithic C implementation (1.x) and doesn't use Perl at all. You should probably upgrade, unless there is a reason 0.0x is better for you?