nss: Include stdio.h and define NULLSTR when debugging is enabled

The NSS module has a compile time option which when enabled makes
ERROR() and DEBUG() print messages onto stderr. But now that the
module no longer links with libvirt, we need to include stdio.h
and define NULLSTR().

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>

tools/nss/libvirt_nss.h

@@ -33,6 +33,8 @@
 
 #if 0
 # include <errno.h>
+# include <stdio.h>
+# define NULLSTR(s) ((s) ? (s) : "<null>")
 # define ERROR(...) \
 do { \
     char ebuf[1024]; \