[gnome-boxes] Fix build with RDP support disabled

[Michael Catanzaro <mcatanzaro src gnome org>]

commit 94ba5bafd7ec03f5cb8ca012a5db3dd952322f40
Author: Michael Catanzaro <mcatanzaro igalia com>
Date:   Wed May 16 13:21:44 2018 -0500

    Fix build with RDP support disabled
    
    We don't have FreeRDP in the GNOME runtime, and it's not my job to add
    it. The option to disable it is broken, so fix it.

 src/rdp-display.vala | 4 ++++
 1 file changed, 4 insertions(+)
---
diff --git a/src/rdp-display.vala b/src/rdp-display.vala
index b161189a..d0d88435 100644
--- a/src/rdp-display.vala
+++ b/src/rdp-display.vala
@@ -1,6 +1,8 @@
 // This file is part of GNOME Boxes. License: LGPLv2+
 using Gtk;
 
+#if HAVE_RDP
+
 private class Boxes.FrdpDisplay: Frdp.Display {
     public override bool authenticate (out string username, out string password, out string domain) {
         username = this.username;
@@ -105,3 +107,5 @@ public override void disconnect_it () {
     public override void send_keys (uint[] keyvals) {
     }
 }
+
+#endif