firefox: fix the drmSupport option

At some point the config flag must have changed from --enable-eme to
--enable-eme=widevine, so it was having no effect.

pkgs/applications/networking/browsers/firefox/common.nix

···
   ++ lib.optional googleAPISupport "--with-google-api-keyfile=ga"
   ++ flag crashreporterSupport "crashreporter"
   ++ flag safeBrowsingSupport "safe-browsing"
-  ++ flag drmSupport "eme"
+  ++ lib.optional drmSupport "--enable-eme=widevine"
 
   ++ (if debugBuild then [ "--enable-debug" "--enable-profiling" ]
                     else [ "--disable-debug" "--enable-release"