strongswan: fix build error 'no rule to make target libtls.la'

In some strongswan configurations, an implicit dependency could be missing.
This commit patches strongswan to force this dependency when needed.

Fixes several autobuild failures, like:
http://autobuild.buildroot.net/results/643f5b33973d5b12ffe6d84254bf3be9ed81d0ad/
http://autobuild.buildroot.net/results/24bcdcb83119e416c92458fa9ff2f5269957a743/



Signed-off-by: Thomas De Schampheleire <thomas.de.schampheleire@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>