I know what I'd do if it were mine.

Like Alk8944 said, I'd shoot it until it showed signs of becoming a bigger problem. If it never did, then so be it. BUT if it did get worse I know how I'd fix it.

I have some aluminum alloy rods that can be melted with a MAP gas torch and will stick to just about any grade of aluminum or magnesium alloy. I'd pack the frame with clay to leave just that mounting tab exposed (to protect the rest of the frame from heat).

Next I'd grind/notch the crack out just a little with a file or Dremel tool and then fill it in with the rod & a torch. Reshape it with hand files & re-drill the hole.

Like Alk8944 said, it is in a non-stressed location and there is no safety concern. All it does is provide a mounting point for the grips. Just not that big or critical of a deal.