Bug #670727: Clarify license of a file to be GPLv2 and GPLv3

gnc-dense-cal.h: License remark was forgotten in r19507, but the author
Josh Sled agreed on GPLv2 and GPLv3 licensing in his gnucash-devel
message on 2010-08-28.

git-svn-id: svn+ssh://svn.gnucash.org/repo/gnucash/trunk@22063 57a11ea4-9604-0410-9ed3-97b8803252fd

src/gnome-utils/gnc-dense-cal.h

@@ -4,7 +4,7 @@
  *                                                                  *
  * This program is free software; you can redistribute it and/or    *
  * modify it under the terms of the GNU General Public License as   *
- * published by the Free Software Foundation, under version 2 of    *
+ * published by the Free Software Foundation, under version 2 and/or version 3 of    *
  * the License.                                                     *
  *                                                                  *
  * This program is distributed in the hope that it will be useful,  *