Kernel Structs are unresolvable

I'm writing a new kernel module with KDevelop. The sources of my kernel are at /usr/src/linux. Compiling my custom module works fine. But if I select in my code a struct defined in the kernel headers, for e.g. sk_buff i get the note in the status bar: "sk_buff : sk_buff (unresolved)" and therefore also autocompletion doesn't work well. What should i configure, that i can also browser through the kernel source?
I tried to make a symbolic link to the kernel headers (ln -s /usr/src/linux/includes/linux linux) in the same directory as the project, but it didn't work as expected. Any idea?