scons: Use the 'png' tag for the PNG library.

Use that instead of checking HAVE_PNG manually.

Change-Id: I70876982c2506bee1173341fa100f5fae2ae4302
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/58709
Maintainer: Gabe Black <gabe.black@gmail.com>
Reviewed-by: Yu-hsin Wang <yuhsingw@google.com>
Reviewed-by: Giacomo Travaglini <giacomo.travaglini@arm.com>
Tested-by: kokoro <noreply+kokoro@google.com>
1 file changed