libusb: update submodule to v1.0.26+3, commit ba69847.

This picks up the "config: allow configure to continue without
pkg-config installed" commit, which fixes builds on Linux.

Also includes these commits on top of v1.0.26:
- "Fix `exit` order to match reverse of `init`"
- "darwin: add missing locking around cached device list cleanup"

Author: flit