strongswan: fix typo in tnccs_20 dependencies

It looks like there is a typing error in dependencies of tnccs_20 in strongswan
configure script. Add a patch to solve it.

Resolve issues detected here:
    http://autobuild.buildroot.net/results/b069e304c42660e7f7eba0b358f95dbaf879df64
    http://autobuild.buildroot.net/results/7355ddbaa213f4fcfd1b6d37b25bed1aab525723



Signed-off-by: Jérôme Pouiller <jezz@sysmic.org>
Tested-by: Thomas De Schampheleire <thomas.de.schampheleire@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>