[buteo-sync-plugins] Fix build breakage

The syncml plugins all build the out-of-process enablers.
If concurrent builds all attempt to output and then use the same
moc / .o files for the shared include files, the build can break.

To avoid this, we put a dependency from the serverplugins target
on the clientplugins target, and ensure that both the clientplugins
and serverplugins are build in order, to avoid concurrent builds.