Fix regression that broke installing desktop file

In commit 5d94db9c79 the desktop file was
moved from src/ to dist/ but the relative path from src/src.pro was
switched to an absolute path from the repository root. This broke
detection of the file from within qmake.

Fix by using the same $DIST_PATH used elsewhere for consistency, which
uses ../dist/.

unixconf.pri

@@ -36,7 +36,7 @@ nogui:systemd {
 
 # Menu Icon
 !nogui {
-    menuicon.files = dist/unix/qbittorrent.desktop
+    menuicon.files = $$DIST_PATH/qbittorrent.desktop
     menuicon.path = $$DATADIR/applications/
     INSTALLS += menuicon