Dockerfile.xenial-gcc9: Install software-properties-common in the correct order

Dockerfile.xenial-gcc9

@@ -77,7 +77,6 @@ ENV PACKAGES \
     sed \
     sudo \
     swig \
-    software-properties-common \
     tar \
     texinfo \
     unzip \
@@ -96,6 +95,7 @@ RUN set -eux; \
     chown -R developer:developer /developer
 
 RUN set -eux; \
+    apt-get -y install software-properties-common; \
     add-apt-repository -y ppa:libretro/testing;
 
 RUN set -eux; \