strongswan: enable plugins-packaged-separately option

OpenWrt ships the plugins in multiple packages that are not all
installed by default. When running tools like 'pki' or 'swanctl',
this currently produces error-level log output for plugins that
are simply not installed, which can alarm users unnecessarily.

Enable the new configure option 'plugins-packaged-separately'
(default: no), added in strongSwan 6.0.5, which sets it to 'yes'.
This lowers the log level of the corresponding messages and adds
a note that the missing plugins may be available in other
packages.

Signed-off-by: Florian Eckert <fe@dev.tdt.de>

net/strongswan/Makefile

@@ -450,6 +450,7 @@ CONFIGURE_ARGS+= \
 	--enable-mgf1 \
 	--enable-mediation \
 	--with-systemdsystemunitdir=no \
+	--with-plugins-packaged-separately=yes \
 	PYTHON="python3" \
 	$(if $(CONFIG_PACKAGE_strongswan-charon-cmd),--enable-cmd,--disable-cmd) \
 	$(if $(CONFIG_PACKAGE_strongswan-mod-gmpdh),--enable-gmpdh,) \