I added Cordova as a component to my iOS project. Adding a custom plugin leads to the error, despite that the plugin works in a Cordova-only project:
\'CDVPl
Click on the project icon in the Project Navigator,
select your Project, then select the "Build Settings" tab
Enter "Header Search Paths" in the search field
Add "$(CORDOVALIB)/Classes" and check the Recursive checkbox - for the Header Search Paths value
optional
Add "-all_load" and "-Obj-C" - for the Other Linker Flags value
see detailed description (step 17)