the libc6-dev package is automatically installed as soon as you install a C-compiler. This package also contains a directory with the Kernel's header files (/usr/include/linux) because any system/linux specific programming depends on them.
The problem is, that these header files are really outdated (afair kernel version 2.4.9) and don't fit to your custom kernel. Simply do the following:
ln -s linux-2.4.21-rc2 linux
mv linux linux.old
ln -s /usr/src/linux/include/linux .
...and your header files are up to date and the nvnet-module will compile (as long as you don't delete your /usr/src/linux