data: Add StartupNotify to desktop file (MR 2)

As a GTK app it supports startup notifications just fine and this allows
phosh to show a proper splash screen while megapixels is starting.

Reference: https://developer-old.gnome.org/integration-guide/stable/startup-notification.html.en

data/org.postmarketos.Megapixels.desktop

@@ -6,3 +6,4 @@ Type=Application
 Categories=GTK;
 Icon=org.postmarketos.Megapixels
 X-Purism-FormFactor=Workstation;Mobile;
+StartupNotify=true