Use FILESEXTRAPATHS_prepend for correct assignment

This fixes the following warning using openembedded-core master:
```
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
```