This fixes a build problem on FreeBSD which was raised by /u/1s44c in [1].
The problem with the original code that the "export PYTHONPATH" is
interpreted by gmake as an improperly indented command for the preceding
target.
Indentation, while it would surmounting the immediate build problem on FreeBSD,
is not the correct solution, since the export is intended to apply to further
command sequences and not to be a associated to the particular target
listed there.
Accordingly, it is moved up which has been verified by inspection to result
in a clean Makefile on FreeBSD and Linux.
[1] https://np.reddit.com/r/BitcoinABC/comments/6yuy53/bitcoin_abc_ubuntu_repo_has_been_updated_to_serve/dms6ip3/