Add pkg-config directory to .gitignore

https://docs.fedoraproject.org/en-US/Fedora_Contributor_Documentation/1/html/Software_Collections_Guide/sect-Software_Collection_pc_Files_Support.html

It's already included in Debian packaging:
http://anonscm.debian.org/gitweb/?p=tahoe/tahoe.git;a=commitdiff;h=7d6a8e3af50c7baea1a3eff721d473be9ec24f5a