fullscreen-shell: Add missing license tag

Although it would probably default to the license at the root of the
repository anyway, it's best to be explicit about it, and also be
consistent with the other extensions.

The copyright holders have been assembled from git history and the
README.

Signed-off-by: Johan Klokkhammer Helsing <johan.helsing@qt.io>
Acked-by: Jason Ekstrand <jason@jlekstrand.net>

unstable/fullscreen-shell/fullscreen-shell-unstable-v1.xml

@@ -1,6 +1,31 @@
 <?xml version="1.0" encoding="UTF-8"?>
 <protocol name="fullscreen_shell_unstable_v1">
 
+  <copyright>
+    Copyright © 2016 Yong Bakos
+    Copyright © 2015 Jason Ekstrand
+    Copyright © 2015 Jonas Ådahl
+
+    Permission is hereby granted, free of charge, to any person obtaining a
+    copy of this software and associated documentation files (the "Software"),
+    to deal in the Software without restriction, including without limitation
+    the rights to use, copy, modify, merge, publish, distribute, sublicense,
+    and/or sell copies of the Software, and to permit persons to whom the
+    Software is furnished to do so, subject to the following conditions:
+
+    The above copyright notice and this permission notice (including the next
+    paragraph) shall be included in all copies or substantial portions of the
+    Software.
+
+    THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+    IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+    FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT.  IN NO EVENT SHALL
+    THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+    LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING
+    FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER
+    DEALINGS IN THE SOFTWARE.
+  </copyright>
+
   <interface name="zwp_fullscreen_shell_v1" version="1">
     <description summary="displays a single surface per output">
       Displays a single surface per output.