Install firefox's native messaging host to LIBDIR

Summary:
0d92aba10fd9402c184ef77706a87efa0d436c3e changed this from a hard
coded value (/usr/lib/...) to a KDE_INSTALL_ variable (yay!), but
used an incorrect one.

Test Plan: Extension communicates with the host again

Reviewers: Plasma, bshah, broulik, davidedmundson

Subscribers: plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D11498