xapi-storage-script: enable building from tarball
xapi-storage-script: enable building from tarball

The merge to this repository forced git to be present. Allow a fallback
mechanism when the package is being built from a tarball.

Signed-off-by: Pau Ruiz Safont <9c4aad32b049b39a74875643d7fe76c19fd4f9a1@citrix.com>

xapi-project/message-switch,xapi-project/message-switch,xapi-project/message-switch