Use system copy of qtsingleapplication.

--- a/musique.pro
+++ b/musique.pro
@@ -18,7 +18,7 @@
 
 QT += network sql widgets
 
-include(src/qtsingleapplication/qtsingleapplication.pri)
+CONFIG += qtsingleapplication
 include(src/http/http.pri)
 include(src/tags/tags.pri)