This fixes an observed failure to detect madvise() on Linux.

To avoid similar issues, all other tests that use NULL but don't already
have stddef.h (or another header that is defined to provide NULL,
such as stdio.h, unistd.h, or time.h) are also fixed.
Signed-off-by: NScott Wood <scottwood@freescale.com>
Signed-off-by: NBlue Swirl <blauwirbel@gmail.com>