Do not update when there is no change

I'm adding a rule which regenerates ninja files to kati:

With this regeneration rule, unnecessary re-generation will happen
when these .mk files are updated even if there are no changes in
their contents. With this patch, these .mk files are updated only
when the contents will be actually changed.

Change-Id: I4c796f9454502f6bb25019b3806ca577ea5258c1
2 files changed