Whitelist webrtc peerconnection_client in build/config/linux/gtk/BUILD.gn

This is a simple src-side dependency required by
https://codereview.webrtc.org/2563203002

BUG=668446

Review-Url: https://codereview.chromium.org/2565063002
Cr-Original-Commit-Position: refs/heads/master@{#437758}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: bf4f82b8448d70608cb54c7e5891f2a52e570d0b

config/linux/gtk/BUILD.gn

@@ -23,6 +23,7 @@ group("gtk") {
     "//remoting/host/it2me:remote_assistance_host",
     "//remoting/host:remoting_me2me_host_static",
     "//remoting/test:it2me_standalone_host_main",
+    "//webrtc/examples:peerconnection_client",
   ]
 
   if (use_gtk3) {