scripts/xldd: install only when shared libs enabled

When the toolchain has no support for shared libraries, there is no
point in installing the cross-ldd helper.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@anciens.enib.fr>

 reporter_name="Nye Liu"

 reporter_url="http://sourceware.org/ml/crossgcc/2008-10/msg00046.html"

 reporter_comment=""