ldd: define the "unused" variable

The "unused" variable could be use unitialized, which is an issue if ldd
is ran with "-u". Fix that by defining the variable to an empty value,
just like it is already done for the bind_now, warn and verbose
variables.

Reported-by: Johan Palmqvist <johan.palmqvist@gmail.com>
Reviewed-by: Sam James <sam@gentoo.org>

elf/ldd.bash.in

@@ -30,6 +30,7 @@ RTLDLIST=@RTLD@
 warn=
 bind_now=
 verbose=
+unused=
 
 while test $# -gt 0; do
   case "$1" in