systemd-compat-units.bb: fix postinstall script

systemd-compat-units.bb: fix postinstall script

This fixes an issue running "opkg upgrade" on a system with systemd
(and when there is an update to "systemd-compat-units",
for example between yocto 4.2.2 and 4.2.3):

//var/lib/opkg/info/systemd-compat-units.postinst: cd: line 3: can't cd to /etc/init.d: No such file or directory

The existence of /etc/init.d is now tested
without causing an error if doesn't exist.

Fixes [YOCTO #15292]

Signed-off-by: Michael Opdenacker <michael.opdenacker@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
(cherry picked from commit 0330331a1386fd2a34b410a7f62b29bfc8dc23c4)
Signed-off-by: Steve Sakoman <steve@sakoman.com>