data: Make feedback support detectable for phosh (MR 24)

Settings apps that allow to tweak feedback per app need a way to detect
if the app supports this. For phosh this happens via
`X-Phosh-UsesFeedback` in the desktop file and results in the app
showing up in the notification settings¹.

1) https://gitlab.gnome.org/guidog/phosh-mobile-settings/-/raw/main/screenshots/feedback.png

data/org.postmarketos.Megapixels.desktop

@@ -6,4 +6,5 @@ Type=Application
 Categories=GTK;Photography;Graphics;
 Icon=org.postmarketos.Megapixels
 X-Purism-FormFactor=Workstation;Mobile;
+X-Phosh-UsesFeedback=true
 StartupNotify=true