Pass export privkey DER compression flag correctly

Summary:
By passing a bitfield where a boolean was expected, the result was
always compressed. Fix this.

This is a backport of Core PR14195

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D8344