Move get_srcdir to where it is used

Summary:
Move get_srcdir function to cdefs.py, it's only
usage location.

Test Plan:

./test/functional/test_runner.py

Reviewers: #bitcoin_abc, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Subscribers: jasonbcox, teamcity

Differential Revision: https://reviews.bitcoinabc.org/D1500