vboot-utils: ssize_t needs unistd.h

Fixes
http://autobuild.buildroot.net/results/95c8708411fbb116f313a0fdeccacf92cf9e3d8d/
http://autobuild.buildroot.net/results/cf4aa60f9fc9c48ff13d3233994b77b7c0950afe/
http://autobuild.buildroot.net/results/6f7dcafde693f06dfc88c922c5f8dd70ae2e30b5/
http://autobuild.buildroot.net/results/a2e95784d32c8842f6947801fd0d8c48f68b84e2/
http://autobuild.buildroot.net/results/f69ae4c9eee27df43389ae76048fb4ddd5a3eca6/



Signed-off-by: Alex Suykov <alex.suykov@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>