Add gcc as a build-package

Adds gcc as a build package to work around a recently introduced
snapcraft bug (https://bugs.launchpad.net/snapcraft/+bug/1883392) which
causes the snap to fail to build due to not having gcc for the gnome
extensions.

Signed-off-by: Billy Olsen <billy.olsen@gmail.com>

snap/snapcraft.yaml

@@ -75,6 +75,9 @@ parts:
     override-build: |
       snapcraftctl build
       mv swing/build/jar/OpenRocket.jar $SNAPCRAFT_PART_INSTALL/OpenRocket.jar
+    build-packages:
+      # Add gcc as a build package due to bug https://bugs.launchpad.net/snapcraft/+bug/1883392
+      - gcc
     stage-packages:
       - openjdk-11-jre
       - ca-certificates