openswan: fix older gcc build failure

Older gcc versions (< 4.4) don't accept cpp as -Werror / -Wno-error
parameters.
Disable it since it's not yet a default with any version and we don't do
-Werror by default. Fixes:
http://autobuild.buildroot.net/results/977/9779215c4dfb68dc105f27097246a97851b1f6c2/



Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>