alsamidisrc: fix compiler warning with clang 10

alsamidisrc: fix compiler warning with clang 10

```
../subprojects/gst-plugins-base/ext/alsa/gstalsamidisrc.c:201:54: error: converting the result of '<<' to a boolean always evaluates to false [-Werror,-Wtautological-constant-compare]
  snd_seq_ev_schedule_real (&ev, alsamidisrc->queue, SND_SEQ_TIME_MODE_ABS,
                                                     ^
/usr/include/alsa/seq_event.h:215:34: note: expanded from macro 'SND_SEQ_TIME_MODE_ABS'
```

The ALSA API expects 0 or 1 here and will then add the flags accordingly,
and that's also what other code using this API does.

Part-of: <https://gitlab.freedesktop.org/gstreamer/gst-plugins-base/-/merge_requests/777>