kvm tools: Fix build breakage due to <linux/stddef.h> changes

kvm tools: Fix build breakage due to <linux/stddef.h> changes

Commit 2084c24 ("do not export kernel's NULL #define to userspace") broken KVM
tool build:

  FYI:

   CC       framebuffer.o
  In file included from include/kvm/framebuffer.h:5:0,
                from framebuffer.c:1:
  ../../include/linux/list.h: In function ‘INIT_HLIST_NODE’:
  ../../include/linux/list.h:572:12: error: ‘NULL’ undeclared (first use in this function)
  ../../include/linux/list.h:572:12: note: each undeclared identifier is reported only once for each function it appears in
  ../../include/linux/list.h: In function ‘hlist_move_list’:
  ../../include/linux/list.h:657:15: error: ‘NULL’ undeclared (first use in this function)
  make: *** [framebuffer.o] Error 1

  due to this upstream commit:

   2084c24a8141 do not export kernel's NULL #define to userspace

Fix that.

Reported-by: Ingo Molnar <mingo@kernel.org>
Signed-off-by: Pekka Enberg <penberg@kernel.org>