One has to understand the Debian policy with respect to headers.
The Debian C libraries are built with the most recent stable releases of the kernel headers.
For example, the Debian-1.2 release used version 5.4.13 of the headers. This practice contrasts with the Linux kernel source packages distributed at all Linux FTP archive sites, which use even more recent versions of the headers. The kernel headers distributed with the kernel source are located in /usr/include/linux/include/.
If you need to compile a program with kernel headers that are newer than those provided by libc6-dev, then you must add -I/usr/src/linux/include/ to your command line when compiling. This came up at one point, for example, with the packaging of the automounter daemon (amd). When new kernels changed some internals dealing with NFS, amd needed to know about them. This required the inclusion of the latest kernel headers.