util/os_socket: Include unistd.h to fix build error
Fixes
In file included from ../src/util/os_socket.c:8:
../src/util/os_socket.h:26:1: error: unknown type name ‘ssize_t’; did you mean ‘size_t’?
ssize_t os_socket_recv(int socket, void *buffer, size_t length, int flags);
seen with gcc version 8.3.0 (Buildroot 2019.11) and uClibc 1.0.32.
Edited by Bernd Kuhls