configure.ac: Update comment in OSS includes check.

configure.ac: Update comment in OSS includes check.

Original commit message from CVS:
* configure.ac:
Update comment in OSS includes check.
* sys/oss/gstossdmabuffer.c:
* sys/oss/gstosshelper.c:
* sys/oss/gstossmixer.c:
* sys/oss/gstossmixertrack.c:
* sys/oss/gstosssink.c:
* sys/oss/gstosssrc.c:
* sys/oss/oss_probe.c:
Don't assume the OSS soundcard.h include is always in
the sys/ directory. Instead, use the existing defines
from config.h to include the right file. Fixes
compilation on OpenBSD 3.8 (#323718).