configure: Add libusb-1.0 header bug workaround

Force the libusb-1.0 include dir to be treated as a system include path
which suppresses the cast-align warnings otherwise failing the build on
some platforms.

Change-Id: I738cfd793052b8772731d2feeef968ee893bd4bd
Signed-off-by: Andreas Fritiofson <andreas.fritiofson@gmail.com>
Reviewed-on: http://openocd.zylin.com/1534


Tested-by: jenkins
Reviewed-by: Spencer Oliver <spen@spen-soft.co.uk>