scripts: Fix gen-version-header

If you mount the osv/build directory to another fast disk device, make will
complaint:

$ make
   make[1]: Entering directory
   `/home/asias/src/cloudius-systems/osv/build/release'
     GEN gen/include/osv/version.h
   fatal: Not a git repository (or any parent up to mount point
   /home/asias/src/cloudius-systems/osv/build)
   Stopping at filesystem boundary (GIT_DISCOVERY_ACROSS_FILESYSTEM not
   set).

Fix it by making scirtps/osv-version.sh using --git-dir.

Signed-off-by: Asias He <asias@cloudius-systems.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>