[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