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)

     1.1 --- a/config/libc.in	Tue Nov 09 10:44:29 2010 +0100
     1.2 +++ b/config/libc.in	Tue Nov 23 21:35:24 2010 +0100
     1.3 @@ -84,6 +84,7 @@
     1.4  config LIBC_XLDD
     1.5      bool
     1.6      prompt "Install a cross ldd-like helper"
     1.7 +    depends on SHARED_LIBS
     1.8      depends on ! BARE_METAL
     1.9      default y
    1.10      help