tools/liblockdep: Fix linker error in case of cross compile

If we try to cross compile liblockdep, even if we set the CROSS_COMPILE variable
the linker error can occur because LD is not set with CROSS_COMPILE.
This patch adds "LD" can be set automatically with CROSS_COMPILE variable so
fixes linker error problem.

Signed-off-by: Eunbong Song <eunb.song@samsung.com>
Signed-off-by: Sasha Levin <sasha.levin@oracle.com>