package/alsa-utils: fix systemd build with pkgconf 1.5.3

Since the bump to 1.5.3, pkgconf prepends the sysroot to all absolute
paths found in the .pc file. This is correct when the paths refer to
something in STAGING_DIR (e.g. libdir, includedir), but not when it
refers to something used for the target.

alsa-utils uses the systemdsystemunitdir variable from systemd.pc to
decide where to install things. Since DESTDIR is prepended to the
install destination, this will end up in the wrong location.

Until a better solution is found in pkgconf, pass the
systemdsystemunitdir to use explicitly instead of relying on systemd.pc.

Fixes:
 - http://autobuild.buildroot.org/results/d8ad140ae52b4fe8e153de3835f3f17e92b58e53



Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>