Fix Linux 32-bit compilation after sysroot switch.

The roll in https://codereview.webrtc.org/1713493002/
made us start using the Chromium sysroot images for libraries instead
of system libraries. This caused Linux 32-bit builds to break with
an error like this:
../../webrtc/examples/peerconnection/client/linux/main_wnd.cc:82:46: error: missing sentinel in function call [-Werror,-Wsentinel]
      "List Items", renderer, "text", 0, NULL);
                                             ^
                                             , nullptr
/usr/include/gtk-2.0/gtk/gtktreeviewcolumn.h:128:25: note: function has been explicitly marked sentinel here
GtkTreeViewColumn      *gtk_tree_view_column_new_with_attributes (const gchar             *title,
                        ^
1 error generated.

This CL suppresses this warning to green up the bots.

TBR=niklase@webrtc.org

Review URL: https://codereview.webrtc.org/1710083003 .

Cr-Commit-Position: refs/heads/master@{#11674}

webrtc/webrtc_examples.gyp

@@ -138,6 +138,11 @@
                 ],
               },
             }],  # OS=="linux"
+            ['OS=="linux" and target_arch=="ia32"', {
+              'cflags': [
+                '-Wno-sentinel',
+              ],
+            }],  # OS=="linux" and target_arch=="ia32"
           ],  # conditions
         },  # target peerconnection_client
       ], # targets