Right, that isn't normal.
If I follow your link to the backtrack-linux.org's wiki page on "BT5 kernel sources" correctly then you need to follow its instructions to get their version of headers.
Apparently prepare-kernel-sources is a script or some such. And it looks like you are suppose to run eveything as root. So if you have sudo first run the script:
Then change directory to /usr/src/linux:
Now copy recursively whatever the script generated in include/generated/ into include/linux/:
sudo cd /usr/src/linux
And I guess you now have their equivalent of kernel headers.
sudo cp -rf include/generated/* include/linux/