Document the GDB 9.1 release in gdb/ChangeLog

gdb/ChangeLog:

	GDB 9.1 released.
diff --git a/gdb/ChangeLog b/gdb/ChangeLog
index 5448f83..07353b7 100644
--- a/gdb/ChangeLog
+++ b/gdb/ChangeLog
@@ -1,3 +1,7 @@
+2020-02-08  Joel Brobecker  <brobecker@adacore.com>
+
+	GDB 9.1 released.
+
 2020-02-05  Iain Buclaw  <ibuclaw@gdcproject.org>
 
 	PR gdb/25190: