linux-headers-4.19.0-21-common - 4.19.249-2 main

This package provides the common kernel header files for Linux kernel
version 4.19.0-21, generally used for building out-of-tree kernel modules.
To obtain a complete set of headers you also need to install the
linux-headers-4.19.0-21-(flavour) package, matching the flavour of the
kernel you intend the build for.