[Chore] Ignore cmake build directories

Summary:
Prevent build directories from
accidently being added

Test Plan: N/A

Reviewers: geekbrother, karol-bisztyga, ashoat

Reviewed By: ashoat

Subscribers: ashoat, palys-swm, Adrian, atul, yayabosh

Differential Revision: https://phab.comm.dev/D4487