[fix-nonhinting] Changing `maxp` `maxSizeOfInstructions` when adding `prep` table

Hello!

I'd like to preface this by saying that i'm unsure whether or not this needs to be implemented/fixed, but since the `maxp` table handles memory requirements I was thinking that it might be important to get right.

#### Issue description

When I run `gftools fix-unhinted` a `prep` table is added to the font but the `maxSizeOfInstructions` remains unchanged. The `prep` table generated for me has 7 lines(bytes) of instructions and thus this should reflect in the `maxp` table

Interested to know your thoughts and thanks in advance for your help!