control: Add png dependency

Examples require libpng in upstream.
2 jobs for f/deb_deps in 2 minutes and 46 seconds (queued for 1 second)
Status Job ID Name Coverage
  Build
passed #1718
librem5
build-with-xwayland

00:01:24

passed #1719
librem5
build-without-xwayland

00:01:22