package/openvpn: use make install instead of custom install step

package/openvpn: use make install instead of custom install step

Commit 7105e65cd6b8f857bab54e4c0a8c57da776b0564 ("package/openvpn:
adds target install of systemd unit files") added the installation of
systemd unit files. But in fact, they can be installed by openvpn's
build system. It was simply not working due to the custom install step
implemented in openvpn.mk.

So instead, let's have the autotools-package infra call "make
install", which properly installs everything that's needed for
openvpn, including systemd units, but also plugins, etc.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>