Fix SEPA plugin installation

Summary:
add_library() may add a "lib" prefix to the plugin's filename, which causes it not to be found.
Using kde4_add_plugin() instead again fixes that.

This change to add_library() was (accidentally?) backported from the KF5 version in commit c78183f18201 .

BUG: 386547
FIXED-IN: 4.8.2

Intended for the 4.8 branch only, of course.

Test Plan:
The plugin is again installed as konlinetasks_sepa.so here (like in 4.8.0, and also the KF5 port).
The error message that the plugin could not be found does not occur anymore, kmymoney prints

onlineTask available "org.kmymoney.creditTransfer.sepa"

to stdout now.

Haven't tested the actual functionality though.

Reviewers: KMyMoney, habacker

Reviewed By: KMyMoney, habacker

Subscribers: tbaumgart

Tags: KMyMoney

Differential Revision: https://phabricator.kde.org/D9545