gpu: Add explicit dependency on X11/Xext when building with X11.

gpu/gpu_config.gypi

 # Copyright (c) 2012 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 {
   'dependencies': [
     '../third_party/re2/re2.gyp:re2',
     '../base/base.gyp:base',
     '../ui/gl/gl.gyp:gl',
   ],
@@ skipping 47 matching lines @@
       'msvs_disabled_warnings': [ 4267, ],
     }],
     ['OS=="win" and branding=="Chrome"', {
       'sources': [
         '../third_party/amd/AmdCfxPxExt.h',
         '../third_party/amd/amd_videocard_info_win.cc',
       ],
     }],
     ['OS=="linux" and use_x11==1', {
       'dependencies': [
+        '../build/linux/system.gyp:x11',
+        '../build/linux/system.gyp:xext',
         '../build/linux/system.gyp:libpci',
         '../third_party/libXNVCtrl/libXNVCtrl.gyp:libXNVCtrl',
       ],
     }],
   ],
 }