dist: ensure the license text is included

The autotools pick up COPYING file automatically.

Change-Id: Ib5c7b479fa66291be987230f102ff391f4902988
Related: OS#6227
diff --git a/gpl-3.0.txt b/COPYING
similarity index 100%
rename from gpl-3.0.txt
rename to COPYING
diff --git a/contrib/gapk.spec.in b/contrib/gapk.spec.in
index e5d06ba..e0466f4 100644
--- a/contrib/gapk.spec.in
+++ b/contrib/gapk.spec.in
@@ -128,7 +128,7 @@
 %postun -n libosmogapk%{sover} -p /sbin/ldconfig
 
 %files
-%doc gpl-3.0.txt
+%doc COPYING
 %{_bindir}/osmo-gapk
 
 %if 0%{with_gsmhr}