Always take GCC's definition of NULL.

There's a (bad) definition of NULL in <linux/stddef.h>, and this
definition of NULL, and the One True definition in the <stddef.h> provided
by the compiler. This change at least kills one of the bad duplicates.
Killing the <linux/stddef.h> one is harder, because it's in a generated

Change-Id: Iea4ccb12d6758199f312ea9cd753b84322d5c471
1 file changed