openswan: needs host-flex

Although parser.l is shipped it seems it might get regenerated from
parser.y sometimes and an old installed flex might not be good enough.
Fixes:
http://autobuild.buildroot.net/results/e59/e5970eae28d01503d89a5d4e593711d7a27da8b5/



Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>