Makefile: Mark unsupported libraries for DOS (DJGPP)

DOS will probably never get USB support.

Change-Id: I6bcc67dc35740b58752663bfcb1c947ba7f70581
Signed-off-by: Nico Huber <nico.h@gmx.de>
Reviewed-on: https://review.coreboot.org/c/flashrom-stable/+/73771
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
1 file changed