util-docker: Add gdb to gcn-gpu Dockerfile

gdb is generally useful.

Change-Id: Ic73822a5f61914cafba0699949dccabc81c03d2a
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/39096
Reviewed-by: Matt Sinclair <mattdsinclair@gmail.com>
Reviewed-by: Jason Lowe-Power <power.jg@gmail.com>
Reviewed-by: Bobby R. Bruce <bbruce@ucdavis.edu>
Reviewed-by: Kyle Roarty <kyleroarty1716@gmail.com>
Maintainer: Matt Sinclair <mattdsinclair@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>

util/dockerfiles/gcn-gpu/Dockerfile

@@ -40,7 +40,8 @@ RUN apt-get update && apt-get install -y --no-install-recommends \
     libboost-filesystem-dev \
     libboost-system-dev \
     libboost-dev \
-    libpng12-dev
+    libpng12-dev \
+    gdb
 
 # Use python 3.9 by default
 RUN update-alternatives --install /usr/bin/python3 python3 /usr/bin/python3.9 1