Fixed this build failure building raylib:
http://www.cpantesters.org/cpan/report/a069fade-0e1f-11e8-a1cf-bb670eaac09d

Functionality is untested, but it now compiles on NetBSD. A quick google search for
libossaudio OpenBSD indicates that linking to libossaudio would be required there too.

raylib issue: raysan5/raylib#463