Add dummy media_keys file

This allows Linux (and MacOS) to compile correctly again. The previous
behavior was an intended middle step.

src/media_keys/media_keys_dummy.cpp

@@ -0,0 +1,25 @@
+#include "media_keys.h"
+
+namespace keyboardinput
+{
+void sendMediaNextSong()
+{
+    // dummy
+}
+
+void sendMediaPreviousSong()
+{
+    // dummy
+}
+
+void sendMediaPausePlay()
+{
+    // dummy
+}
+
+void sendMediaStopSong()
+{
+    // dummy
+}
+
+} // namespace keyboardinput