Add `Chat` to Linux app categories

most dists inc Arch have Chat and IRCClient,
though the latter isn't exactly correct so missing it

package.json

@@ -158,7 +158,7 @@
     ],
     "linux": {
       "target": "deb",
-      "category": "Network;InstantMessaging;",
+      "category": "Network;InstantMessaging;Chat",
       "maintainer": "support@riot.im",
       "desktop": {
         "StartupWMClass": "riot-web"