Use CMAKE_INSTALL_BINDIR for dbus service generation

Summary: The old implementation used CMAKE_INSTALL_PREFIX for determining the installation directory. The target is installed by using KF5_INSTALL_TARGETS_DEFAULT_ARGS and that internally uses CMAKE_INSTALL_BINDIR. In my case I used a distribution (Nixos) where CMAKE_INSTALL_BINDIR is not a sub directory of CMAKE_INSTALL_PREFIX and thus Dbus could not find the kwallet5d executable.

Reviewers: apol

Reviewed By: apol

Subscribers: Frameworks

Tags: Frameworks, KSecrets Service

Differential Revision: https://phabricator.kde.org/D6519