package/lvm2: fix installation of systemd units

Since we bump the version in commit 80997acd (package/lvm2: bump
version to 2.03.12), the installation of systemd units is no longer
functional without a full installation.

As Pascal puts it: the systemd service files don't make a whole lot of
sense when there isn't a full lvm2 install.

Move the conditional block that install system units, so that it only
occurs when we do a full installation.

Fixes;
    http://autobuild.buildroot.org/results/f47/f470ffb55625e2639cecde713442550eb532d0d7/
    http://autobuild.buildroot.org/results/954/9547929292e81671fbe3a5b4bbc87a6424edb1ca/
    http://autobuild.buildroot.org/results/303/30359c351a6ce2f9139494a531e036f0b0406ccf



Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>
Co-Developped-by: Pascal de Bruijn <p.debruijn@unilogic.nl>
Cc: Pascal de Bruijn <p.debruijn@unilogic.nl>
Cc: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>