When exporting a feature, or a product based on features you get an error like:
Processing inclusion from feature <your feature here>: Bundle org.eclipse.core.net.linux.x86 <version here> failed to resolve.:
This is caused by a missing window system/operating system/architecture filter on the fragment’s inclusion in the feature.xml. This can easily be set in the Plug-ins Tab of the feature editor by selecting the fragment on the left and specifying the Operating System, Window System and Architecture on the right.
Quoted from Eclipse PDE Build FAQ.