[SPARK-19562][BUILD] Added exclude for dev/pr-deps to gitignore

## What changes were proposed in this pull request?

Just adding a missing .gitignore entry.

## How was this patch tested?

Entry added, now repo is not dirty anymore after running the build.

Author: Armin Braun <me@obrown.io>

Closes #16904 from original-brownbear/SPARK-19562.