strongswan: unavailable for static builds

Uses dlopen in plugin_loader (not optional), fixes:
http://autobuild.buildroot.net/results/601/601d8dc1654d8733db49b195139e12437663034c/



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