[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: If134b803b0766aefdbf22aabc09a5e8018809139
Reviewed-on: https://fuchsia-review.googlesource.com/c/third_party/pyyaml/+/494880
Reviewed-by: Shai Barack <shayba@google.com>
Reviewed-by: David Fang <fangism@google.com>