[build] Add py_srcs.gni

Mako is imported by a few mesa build actions. Those actions can add
these srcs to their inputs to improve hermeticity.

Change-Id: Ie268a0cdb2039a900eeb3c9b555b94bdcd1997c2
Reviewed-on: https://fuchsia-review.googlesource.com/c/third_party/mako/+/503619
Reviewed-by: Anirudh Mathukumilli <rudymathu@google.com>
Reviewed-by: David Fang <fangism@google.com>
2 files changed