Bug#1021256: gtkmm4: autopkgtest failure: Failed to locate “gtk-logo.webm” in any source directory

[Jeremy Bicha]

Control: severity -1 important

Since gtkmm4.0 is new to Debian, a broken autopkgtest isn't a
regression and doesn't block migration to Testing.

I've fixed the one line seen in the autopkgtest failure, but…

We need to either port the minimal gtkmm app to gtkmm4 or drop it and
just use the included demos apps to test whether simple gtkmm apps
build and work.

Thank you,
Jeremy Bicha