I've had good results using a combination of plastic epoxy & steel.

You will have to remove the headlight, drill a small hole through the broken tab(lengthwise, on the skinny plane, use a vice), then hold the tab in place(where it was before it broke) & drill the hole through the headlight housing.
Then find a wire brad or small finish nail(or paper clip or ?) that will fit the hole you drilled, "dry fit" the parts together, cut the nail to length so it doesn't protrude into the housing, then mix up the epoxy, coat the broken surfaces & the nail with epoxy, & assemble. Use the extra epoxy (there is always extra) to build up around the break & let it dry for 10-12 hrs.
If you want to get really fancy, you could incorporate fiberglass cloth into the build up around the break, & saturate that with the epoxy too.

If that doesn't work, maybe a small bungee would be an option?