nbd: use BPN in Files

Fixed when build with multilib:
lib32-nbd-3.11: lib32-nbd: Files/directories were installed but not shipped in any package:
  /usr/sbin/nbd-client
  /usr/bin/nbd-trdump
  /usr/bin/nbd-server
Please set FILES such that these items are packaged. Alternatively if they are unneeded, avoid installing them or delete them within do_install.
lib32-nbd: 3 installed and not shipped files. [installed-vs-shipped]

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>