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)

 # Default test suite configuration

 # Default DejaGnu configuration

 DG_TOOLNAME = gcc

 DG_TARGET_HOSTNAME = 127.0.0.1

 DG_TARGET_USERNAME = root

 # Default tests

 DG_C_TESTS = 

 DG_CPP_TESTS =