exclude C source files from non-darwin targets

backend/app.go

@@ -235,12 +235,14 @@ func (a *App) setupMPRIS(mprisAppName string) {
 
 func (a *App) setupMPMedia() {
 	a.MPMediaHandler = NewMPMediaHandler(a.Player, a.PlaybackManager)
+	if a.MPMediaHandler != nil {
 		a.MPMediaHandler.ArtURLLookup = func(coverID string) (string, error) {
 			// artwork needs to be in cache before playback, so cover is retrieved in advance.
 			a.ImageManager.GetCoverThumbnail(coverID)
 			return a.ImageManager.GetCoverArtUrl(coverID)
 		}
 	}
+}
 
 func (a *App) LoginToDefaultServer(string) error {
 	serverCfg := a.ServerManager.GetDefaultServer()

backend/mpmediabridge.h

@@ -1,3 +1,5 @@
+//go:build darwin
+
 /**
  * mpmediabridge.h
  * 

backend/mpmediabridge.m

@@ -1,3 +1,5 @@
+//go:build darwin
+
 #import "mpmediabridge.h"
 
 /**