Only enable git ssb if installed

src/freedombone-image-mesh

     echo "scuttlebot pub started" >> /var/log/${PROJECT_NAME}.log
 
     # git ssb site
-    ln -s /etc/nginx/sites-available/git_ssb /etc/nginx/sites-enabled/git_ssb
+    if [ -f /etc/nginx/sites-available/git_ssb ]; then
+        ln -s /etc/nginx/sites-available/git_ssb /etc/nginx/sites-enabled/git_ssb
+    fi
 }
 
 # whether to reset the identity