Update license headers to explicitly say GPLv2+
diff --git a/Process.c b/Process.c
index 556acf4..70110a0 100644
--- a/Process.c
+++ b/Process.c
@@ -2,7 +2,7 @@
 htop - Process.c
 (C) 2004-2015 Hisham H. Muhammad
 (C) 2020 Red Hat, Inc.  All Rights Reserved.
-Released under the GNU GPLv2, see the COPYING file
+Released under the GNU GPLv2+, see the COPYING file
 in the source distribution for its full text.
 */