LICENSES: Update GFDL invariant text to match official wording

LICENSES: Update GFDL invariant text to match official wording

This was flagged by lintian against the Debian package; the text stating
there are no invariant sections deviates from the official GNU wording.
Update it to match the text at the bottom of:

https://www.gnu.org/licenses/old-licenses/fdl-1.2.en.html

Change-Id: Ie222237a8eede24c1b71218b05e1513b74208a47
Signed-off-by: Jonathan McDowell <noodles@earth.li>
Reviewed-on: http://openocd.zylin.com/5974
Tested-by: jenkins
Reviewed-by: Antonio Borneo <borneo.antonio@gmail.com>