Fix system rtmidi library usage

[dawkagaming]

Hello,

In the newest version it hasn't been changed to inherit the rtmidi dependency when using the system library, so here's the fix 😄

Thanks,
Dawid Kulas
SP9SKA

[aethersdr-agent]

Thanks Dawid — this is a correct, well-targeted fix. MidiControlManager.h includes RtMidi.h, and it's a PUBLIC header of aethercore (the executable and midi_settings_test compile against it). The bundled branch already accounts for this with target_include_directories(aethercore PUBLIC third_party/rtmidi). The system branch was the asymmetric one: linking PkgConfig::rtmidi as PRIVATE kept the imported target's INTERFACE_INCLUDE_DIRECTORIES from propagating, so any aethercore consumer including MidiControlManager.h → RtMidi.h would fail to resolve the header under -DUSE_SYSTEM_RTMIDI=ON. Switching to PUBLIC restores parity with the bundled path and propagates the include path (and link) transitively.

No blocking issues, no polish notes — the change is minimal and matches the existing convention in the adjacent block. Commit is signed, so nothing to flag there.

One thing worth confirming (not a blocker, and outside this diff): the CI matrix likely builds with the default USE_SYSTEM_RTMIDI=OFF, so this path may not be exercised automatically. If you built locally with -DUSE_SYSTEM_RTMIDI=ON before and after, mentioning that in the PR gives maintainers the demonstrated-fix evidence they like to see.

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[dawkagaming]

I checked it locally and with PRIVATE it fails to build and when I changed it to PUBLIC, it worked fine, compiled and ran, so it's okay.

[jensenpat]

Holding for next's week release.