d/a/copyright: Add missing copyright information

Signed-off-by: Ariel D'Alessandro <ariel.dalessandro@collabora.com>

debian/apertis/copyright.yml

+Xext/*:
+  license: MIT
+Xext/Makefile.in:
+  license: FSFULLR
+Xi/*:
+  license: MIT
+Xi/Makefile.in:
+  license: FSFULLR