net: cdc_ncm: add include protection to cdc_ncm.h

This makes it a lot easier to test modified versions

Cc: Alexey Orishko <alexey.orishko@gmail.com>
Signed-off-by: Bjørn Mork <bjorn@mork.no>
Signed-off-by: David S. Miller <davem@davemloft.net>