Fix automatic libofx detection

Summary:
Commit 21338733bacaa53b8c53758d0b2c68e9ef543147 broke the autodetection
of LibOFX. This patch fixes this and allows successful compiliation
while ENABLE_OFXIMPORTER is ON and LibOFX is not being installed.

Test Plan:
Installed LibOFX, ran cmake: OFX was enabled and KMyMoney compiled
Uninstalled LibOFX, ran cmake: OFX was disabled and KMyMoney compiled
Reinstalled LibOFX, ran cmake: OFX was enabled and KMyMoney compiled

Reviewers: wojnilowicz

Reviewed By: wojnilowicz

Differential Revision: https://phabricator.kde.org/D9584