sm.puri.Squeekboard.desktop: make path to Exec= absolute

This uses the same logic from Phosh to render absolute paths in .desktop
files.

data/sm.puri.Squeekboard.desktop.in.in

@@ -0,0 +1,9 @@
+[Desktop Entry]
+Name=Squeekboard
+GenericName=Squeekboard Virtual Keyboard
+Comment=Virtual Keyboard
+Exec=@bindir@/squeekboard
+Terminal=false
+Type=Application
+NoDisplay=true
+Categories=GTK;Utility;