Bump nvda-mathcat (#19960)
### Summary of the issue:
nvaccess/nvda-mathcat#5 added a .gitignore to that submodule so that the
submodule isn't marked dirty by the egg-info directory.
### Description of user facing changes:
None
### Description of developer facing changes:
Git won't show `include/nvda-mathcat` as dirty all the time.
### Description of development approach:
`cd include/nvda-mathcat && git checkout master && git pull && cd ../ &&
git add nvda-mathcat`
### Testing strategy:
None needed
### Known issues with pull request:
None
