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 ci-png in 2 minutes and 53 seconds (queued for 1 second)
latest
Status Job ID Name Coverage
  Build
passed #1891
librem5
build-with-xwayland

00:01:28

passed #1892
librem5
build-without-xwayland

00:01:25