x11: Include X11 dependencies only when building with X11.

BUG=none

Review URL: https://codereview.chromium.org/12563002

git-svn-id: http://src.chromium.org/svn/trunk/src/build@186816 4ff67af0-8c30-449e-8e8b-ad334ec8d88c

linux/system.gyp

@@ -748,7 +748,7 @@
             ],
           },
         }],
-        ['chromeos==1', {
+        ['use_x11==1', {
           'link_settings': {
             'libraries': [ '-lXtst' ]
           }