package/nbd: fix static builds

package/nbd: fix static builds

Fixes:
    http://autobuild.buildroot.org/results/aa8/aa8a1ac35a93e1c8b9fddbc2b5d66ecaa921f31e/ (arm)
    http://autobuild.buildroot.org/results/e5b/e5bd05623dd6c57bb0338592ecd21a5ba0cbe95a/ (arc)
    http://autobuild.buildroot.org/results/e0f/e0fd23eec2dd31bece122d84515b2b7cb14594b8/ (ppc)
    http://autobuild.buildroot.org/results/3a2/3a2d4124f78d4e504e8e961fabe038e570923408/ (mips)
    ...

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>