[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: Ic85d2bbfb12d25b4cebd355839a89ce1dc1ee166
Reviewed-on: https://fuchsia-review.googlesource.com/c/third_party/markupsafe/+/494879
Reviewed-by: David Fang <fangism@google.com>