systemd: Fix syntax overrides

Signed-off-by: Andrei Gherzan <andrei.gherzan@huawei.com>

meta-oniro-core/recipes-core/systemd/systemd_%.bbappend

@@ -6,5 +6,5 @@
 # the project.
 deltask warn_musl
 
-FILESEXTRAPATHS_prepend := "${THISDIR}/${PN}:"
+FILESEXTRAPATHS:prepend := "${THISDIR}/${PN}:"
 SRC_URI += "file://systemd-244-musl-1.2.2.patch"