Fix Linux link errors for some targets by adding -lX11 to gpu.gyp's
dependencies.

Broken out and landed separately from  https://codereview.appspot.com/6826086/

git-svn-id: http://skia.googlecode.com/svn/trunk/gyp@6388 2bbb7eff-a529-9590-31e7-b0007b416f81
diff --git a/gpu.gyp b/gpu.gyp
index 4f02d00..d5223e4 100644
--- a/gpu.gyp
+++ b/gpu.gyp
@@ -214,6 +214,7 @@
             'libraries': [
               '-lGL',
               '-lGLU',
+              '-lX11',
             ],
           },
         }],