[build] Add a GN var for sources

This allows dependents to include sources of this library to improve
correctness of incremental builds.

http://fxrev.dev/c/494239/2 demonstrates how this var is intended to be
used.

Change-Id: I792b81197eb28afc56eceae8eae1c5c9d4dc9282
Reviewed-on: https://fuchsia-review.googlesource.com/c/third_party/jinja2/+/494878
Reviewed-by: David Fang <fangism@google.com>
1 file changed