Dockerfile.xenial-gcc9: Fix typo

Dockerfile.xenial-gcc9

@@ -119,8 +119,8 @@ RUN set -eux; \
     rm -rf /var/lib/apt/lists/*
 
 RUN set -eux; \
-    apt-get update -y;
-    apt-get build-dep -y retroarch;
+    apt-get update -y; \
+    apt-get build-dep -y retroarch; \
     rm -rf /var/lib/apt/lists/*
 
 RUN set -eux; \