zunitc: Fix undeclared identifier 'NULL'

Since v6 release, -Dtest-junit-xml=false build regressed because its
ifdef branch no longer includes stddef.h and thus NULL, either directly
or through another header. Using musl-1.1.22 and llvm-8.0.0.
2 jobs for null in 1 minute and 3 seconds (queued for 5 minutes and 2 seconds)
Status Job ID Name Coverage
  Container Prep
passed #277705
container_prep

00:00:26

 
  Build
passed #277706
build-native-meson

00:00:36