systemtap: Set FILESEXTRAPATHS_prepend rather than FILESPATH.

With the systemtap_git.inc file overriding FILESPATH additional layers
cannot append any new items to SRC_URI.

Signed-off-by: Drew Moseley <drew_moseley@mentor.com>
Merge pull request #9 from drewmoseley/fix-systemtap-filespath

systemtap: Set FILESEXTRAPATHS_prepend rather than FILESPATH.