.gitignore: Don't ignore depends patches

Summary:

Ignoring patches might be useful for those who use git format-patch
often, but in our depends folder we want to keep track of our patches.

Backport of core PR15978.

Test Plan:

NOP();

Reviewers: #bitcoin_abc, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Subscribers: jasonbcox

Differential Revision: https://reviews.bitcoinabc.org/D5632