nbd: fix LIC_FILES_CHKSUM

The file depcomp would be changed during configure, which is not
suitable for LIC_FILES_CHKSUM, there is a COPYING file which is GPLv2,
so use it.

Signed-off-by: Robert Yang <liezhi.yang@windriver.com>
Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>
Signed-off-by: Joe MacDonald <joe_macdonald@mentor.com>
Signed-off-by: Armin Kuster <akuster808@gmail.com>