util: Add build dependency to gcn Dockerfile

src/base/pngwriter.cc requires libpng-dev

Change-Id: I7f009cd8f5cacd64150c06b716b1ce3008832910
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/32474
Maintainer: Bobby R. Bruce <bbruce@ucdavis.edu>
Tested-by: kokoro <noreply+kokoro@google.com>
Reviewed-by: Matt Sinclair <mattdsinclair@gmail.com>

util/dockerfiles/gcn-gpu/Dockerfile

@@ -33,7 +33,8 @@ RUN apt-get update && apt-get install -y --no-install-recommends \
     libssl-dev \
     libboost-filesystem-dev \
     libboost-system-dev \
-    libboost-dev
+    libboost-dev \
+    libpng12-dev
 
 ARG gem5_dist=http://dist.gem5.org/dist/develop