lang/swipl: fix build on !amd64
Including poll.h is guarded behind HAVE_POLL_H, but then it also starts
using poll() and struct pollfd, which fails:
/wrkdirs/usr/ports/lang/swipl/work/swipl-9.2.9/src/os/pl-file.c:2604:25: error: array has incomplete element type 'struct pollfd'
2604 | struct pollfd poll_buf[FASTMAP_SIZE];
| ^
/wrkdirs/usr/ports/lang/swipl/work/swipl-9.2.9/src/os/pl-file.c:2604:10: note: forward declaration of 'struct pollfd'
2604 | struct pollfd poll_buf[FASTMAP_SIZE];
| ^
/wrkdirs/usr/ports/lang/swipl/work/swipl-9.2.9/src/os/pl-file.c:2642:45: error: invalid application of 'sizeof' to an incomplete type 'struct pollfd'
2642 | else if ( !(poll_map = malloc(count*sizeof(*poll_map))) )
| ^~~~~~~~~~~
/wrkdirs/usr/ports/lang/swipl/work/swipl-9.2.9/src/os/pl-file.c:2604:10: note: forward declaration of 'struct pollfd'
2604 | struct pollfd poll_buf[FASTMAP_SIZE];
| ^
/wrkdirs/usr/ports/lang/swipl/work/swipl-9.2.9/src/os/pl-file.c:2644:35: error: invalid application of 'sizeof' to an incomplete type 'struct pollfd'
2644 | memset(poll_map, 0, count*sizeof(*poll_map));
| ^~~~~~~~~~~