CI: Build-depend on libpng

Needed for the screencopy example that will be pulled in with our next
update from upstream's master as of
a7a96d76.
2 jobs for f/gitlab-ci in 2 minutes and 55 seconds (queued for 3 seconds)
Status Job ID Name Coverage
  Build
passed #1895
librem5
build-with-xwayland

00:01:24

passed #1896
librem5
build-without-xwayland

00:01:31