audio/mixxx21: fix build on non-x86

Ports shouldn't optimize for the builder's CPU. It also breaks build on non-x86.

Also include sys/types.h in plugins/soundsourcem4a/soundsourcem4a.h. GCC needs it.

PR: 239322
Approved by: acm (maintainer timeout), tcberner (mentor)
Differential Revision: https://reviews.freebsd.org/D21146