graphics/darktable: Include stddef.h when testing zError() symbol

The CheckZlib.cmake module uses NULL in the test source code to
check for the zError() symbol. This fails to build on FreeBSD 10.3
with the following error:

CheckPrototypeDefinition.c:15:10: error: use of undeclared identifier 'NULL'
  return NULL;
         ^

I don't know the root cause for this failure, but including stddef.h
in this test source code fixes the problem.

PR: 225501
Reported by: cpm@