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

This package provides the common kernel header files for Linux kernel
version 4.19.0-21 with the PREEMPT_RT featureset, 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.