dfu-util: missing includes

[Bruce Cran]

I've been porting dfu-util to work with FreeBSD using the new USB stack
and the libusb 0.1 compatibility layer.  While it builds and runs, I
noticed a few compiler warnings because libusb20_compat01.h doesn't
include unistd.h and stdlib.h: src/main.c needs stdlib.h and unistd.h
while src/sam7dfu.c just needs unistd.h - they should ideally be
explicitly included instead of picking them up from libusb.

-- 
Bruce Cran