fix(example): Fix window icon on wayland

[provokateurin]

Copied from flutter/flutter#154522

Now the icon properly shows up for the flatpak:

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 28.84%. Comparing base (043e219) to head (420e3cf).
Report is 8 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2606   +/-   ##
=======================================
  Coverage   28.84%   28.84%           
=======================================
  Files         370      370           
  Lines      136488   136488           
=======================================
  Hits        39369    39369           
  Misses      97119    97119           

Flag	Coverage Δ		*Carryforward flag
account_repository	98.47% <ø> (ø)
cookie_store	99.48% <ø> (ø)		Carriedforward from 043e219
dashboard_app	96.05% <ø> (ø)
dynamite	31.05% <ø> (ø)		Carriedforward from 043e219
dynamite_end_to_end_test	61.69% <ø> (ø)		Carriedforward from 043e219
dynamite_runtime	85.40% <ø> (ø)		Carriedforward from 043e219
interceptor_http_client	97.18% <ø> (ø)		Carriedforward from 043e219
neon_dashboard	96.05% <ø> (ø)		Carriedforward from 043e219
neon_framework	61.11% <ø> (ø)
neon_http_client	97.50% <ø> (ø)		Carriedforward from 043e219
neon_notifications	100.00% <ø> (ø)		Carriedforward from 043e219
neon_storage	94.66% <ø> (ø)		Carriedforward from 043e219
neon_talk	99.45% <ø> (ø)		Carriedforward from 043e219
nextcloud	24.32% <ø> (ø)		Carriedforward from 043e219
notifications_app	97.40% <ø> (ø)
notifications_push_repository	98.11% <ø> (ø)
sort_box	90.90% <ø> (ø)		Carriedforward from 043e219
talk_app	98.94% <ø> (ø)

*This pull request uses carry forward flags. Click here to find out more.