Dockerfile.xenial-gcc9: Attempt to fix 'apt-get build-dep -y retroarch' error

Dockerfile.xenial-gcc9

@@ -118,6 +118,7 @@ RUN set -eux; \
     rm -rf /var/lib/apt/lists/*
 
 RUN set -eux; \
+    add-apt-repository -y ppa:libretro/testing; \
     apt-get update -y; \
     apt-get build-dep -y retroarch; \
     rm -rf /var/lib/apt/lists/*