install path fixes

• rpm: correct pkgconfig path

pkconfig files get installed to $datarootdir/pkgconfig but rpm expects
them to be at $datadir. This works when $datarootdir==$datadir which is
the case most of the time but will fail when they differ.

• install: make initramfs-tools path static

Since initramfs-tools' path is nothing we can control as it is an
external package it does not make any sense to install zfs additions
anywhere else. Simply use /usr/share/initramfs-tools as path.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Richard Laager <rlaager@wiktel.com>
Signed-off-by: Michael Niewöhner <foss@mniewoehner.de>
Closes #9087