package/ctorrent: fix compilation under musl.

Fixes http://autobuild.buildroot.net/results/2c9/2c9ef771d146ce5b9df82735fdeb62926c84bd9f/



Build failed due to use of ssize_t without including <unistd.h>.

Patch submitted upstream (note: project appears dormant).

Signed-off-by: Brendan Heading <brendanheading@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>