util/os_socket: Include unistd.h to fix build error

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.

Reviewed-by: Eric Engestrom <eric@engestrom.ch>
Fixes: ef5266ebd50e7fa65c56 ("util/os_socket: Add socket related functions.")
Tested-by: Marge Bot <https://gitlab.freedesktop.org/mesa/mesa/merge_requests/3659>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/merge_requests/3659>