os-specific/linux/vmm_clock: mark kernels older than 4.19 as broken

Upstream mentions[1] the oldest tested kernel is 4.19, so mark anything
older as broken.

ZHF: #172160

[1] https://github.com/voutilad/vmm_clock#tested-platforms-and-configs

pkgs/os-specific/linux/vmm_clock/default.nix

@@ -26,6 +26,7 @@ stdenv.mkDerivation rec {
   ];
 
   meta = with lib; {
+    broken = kernel.kernelOlder "4.19";
     description =
       "Experimental implementation of a kvmclock-derived clocksource for Linux guests under OpenBSD's hypervisor";
     homepage = "https://github.com/voutilad/vmm_clock";