Fix apptest build failure on Xenial

Summary:
Due to a missing definition with the Xenial Qt version, ordering of the
headers in apptests.cpp end up being significant.
This only occurs when building with autotools, as CMake does adds the
missing definition (on the command line on through a generated header,
depending on the version).

Test Plan:
On Ubuntu Xenial:

make

Reviewers: #bitcoin_abc, deadalnix, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Subscribers: jasonbcox

Differential Revision: https://reviews.bitcoinabc.org/D4654