[flashrom] Fix feature detection when fdti requires libusb for linking

[Joerg Mayer]

see attached patch. patch is too small to be copyrightable...

 ciao
     Joerg
-- 
Joerg Mayer                                           <jmayer(a)loplof.de>
We are stuck with technology when what we really want is just stuff that
works. Some say that should read Microsoft instead of technology.