doc: Add a file to ldoc blacklist before addint it.

Add `dpi.lua` to config.ld even if it isn't added yet. This is
because the way the test run has it cached in the build dir. A full
rebuild would take too long and timeout on travis for semi-large PRs.

docs/config.ld

@@ -137,6 +137,7 @@ file = {
         '../lib/awful/dbus.lua',
         '../lib/awful/init.lua',
         '../lib/awful/remote.lua',
+        '../lib/awful/screen/dpi.lua',
         '../lib/awful/startup_notification.lua',
         '../lib/awful/mouse/drag_to_tag.lua',
         '../lib/gears/init.lua',