Ninja: comment abitrary kernel version cutoff

Add comment about the entirely arbitrary kernel version
cutoff. It's used to decide whether to use the package manager
or install from pip. It's based on what's needed for DPDK,
so eventually the condition may change to be more specific based
on LISA's need.

lisa/tools/ninja.py

@@ -28,6 +28,9 @@ def can_install(self) -> bool:
 
  def _install(self) -> bool:
  node = self.node
+ # entirely arbitrary kernel version cutoff
+ # to seperate older distros (pre-2020) from newer ones
+ # that package recent versions of ninja.
  if (
  isinstance(self.node.os, Posix)
  and self.node.os.get_kernel_information().version >= "5.15.0"