Distributing kernel-devel built for dom0 to VMs is major PITA, including
problems like copying between different build environments, inability to
install it in Debian etc.
Lets include kernel headers there by default, without any additional
package needed.
FixesQubesOS/qubes-issues#2692
When VM kernel is provided by dom0 (the default in Qubes 3.x), VM
kernel-devel package should match kernel actually installed in dom0.
FixesQubesOS/qubes-issues#2641