As the whitelist define can be set per compilation unit in all
legitimate cases, the checks do not need to be run on every single usb.h
inclusion. This is done for two reaons:
* It is sufficient -- if any user C file includes usb.h, there's already
a good chance that the user is doing something USB related manualy.
(And conversely, the existing examples with boards that happen to pull
in CDC-ACM or CDC-ECM do not include usb.h from an example C file).
* Defining the USB_H_USER_IS_RIOT around legitimate uses of the header
by other headers would allow accidental sidestepping: If a user
includes a legitimate usb.h using header (say, board.h) and just
forgets to include usb.h on their own, their application that'd mess
with USB would still work as usb.h is transitively included, and the
check for custom includes does not trigger.
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