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.

tools/zunitc/src/zuc_junit_reporter.c

@@ -460,6 +460,7 @@ zuc_junit_reporter_create(void)
 
 #else /* ENABLE_JUNIT_XML */
 
+#include <stddef.h>
 #include "zuc_event_listener.h"
 
 /*