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>
(transplanted from 032ae3253a655d859f25a5c2bac1b72051f950b0)

reporter_name="Thomas Petazzoni"

reporter_url="http://www.free-electrons.com/"

reporter_comment="Very basic testing only"