libusb/core.c: bump DISCOVERED_DEVICES_SIZE_STEP to 16

On OpenBSD USB controllers are shown as normal devices, making the
initial limit too small. On a recent machine this value is almost
always exceeded, so bump it.

Change committed to OpenBSD ports tree by Matthias Kilian <kili@openbsd.org>
on Tue Jun 19 21:10:59 2012 UTC

Closes #835

Signed-off-by: Chris Dickens <christopher.a.dickens@gmail.com>