[portmidi] Scope alsa dependency to Linux/Android/{Free,Open}BSD

[fwcd]

When building for a platform like iOS portmidi (erroneously) has a dependency on ALSA. To fix this, this PR updates the platform condition from !(windows | osx) to linux | android | freebsd | openbsd (as per the description that portmidi supports BSDs). Other platforms using alsalib can be added as needed.

An alternative solution would be to exclude iOS, but my gut feeling is that declaring platforms inclusively is a more robust solution here since we avoid regressing on potentially new platforms. Thoughts welcome.

• Changes comply with the maintainer guide.
• SHA512s are updated for each updated download (not applicable).
• The "supports" clause reflects platforms that may be fixed by this new version (not applicable).
• Any fixed CI baseline entries are removed from that file (not applicable).
• Any patches that are no longer applied are deleted from the port's directory (not applicable).
• The version database is fixed by rerunning ./vcpkg x-add-version --all and committing the result.
• Only one version is added to each modified port's versions file.