If you aren't fussy, you can drill it with a 7/32" drill and tap it 1/4-20.
I mention this, because that drill size is commonly found in small, fractional "homeowner" type drill sets.
1/4-20 is a common US bolt size.
If you want to stay Metric, a #8 drill and 6mm tap are required.
You might even do both, just to keep them the same.