Update .gitignore to keep libmctp.pc.in

[antoine-peartree]

File libmctp.pc.in should not be ignored as it is mandatory for the build but not generated automatically

[amboar]

Thanks for the patch @antoine-peartree. Unfortunately a couple of administrative things:

1. Patches for OpenBMC are handled through the Gerrit instance rather than PRs: https://gerrit.openbmc-project.xyz/
2. For better or worse, contributions to OpenBMC require signing a CLA, though this doesn't require copyright assignment: https://github.com/openbmc/docs/blob/master/CONTRIBUTING.md#starting-out

I recognise this is a bunch of overhead for a straight-forward cleanup, so maybe we can consider other options if you don't want to go through those processes.

[antoine-peartree]

As I'm moving to another project soon, I won't be a regular OpenBMC contributer.
If someone already registered to OpenBMC could fix this, that would probably be better.
Thanks

[amboar]

Dealt with here: https://gerrit.openbmc.org/c/openbmc/libmctp/+/66469