update-rc.d: Use _prepend to add to the extras

WARNING: FILESEXTRAPATHS-variable, must always use _prepend (or _append)
type of assignment, and don't forget the colon.
Please assign it with the format of:
  FILESEXTRAPATHS_append := ":${THISDIR}/Your_Files_Path" or
  FILESEXTRAPATHS_prepend := "${THISDIR}/Your_Files_Path:"
in your bbappend file

recipes-fixes/update-rc.d/update-rc.d_angstrom.inc

@@ -1,4 +1,4 @@
-FILESEXTRAPATHS := "${THISDIR}/files"
+FILESEXTRAPATHS_prepend := "${THISDIR}/files:"
 
 SRC_URI += "file://0001-update-rc.d-make-s-a-noop-when-systemd-is-present.patch"