oasis: Include linker flags needed on Debian

This path is needed on Debian, where the Xen libraries are
installed in a non-standard path. It should not do any
harm on CentOS as it is an addition to the library search
path, not a replacement. Ideally the configure script
would fill the path in as appropriate.

Signed-off-by: Euan Harris <euan.harris@citrix.com>