Add missing header include
Add missing header include

NULL is a macro defined in stddef.h and we use it all over the place.

Signed-off-by: Alexander Dahl <9b466094ec991a03cb95c489c19c4d75635f0ae5@lespocky.de>

LeSpocky/libcgi,rafaelsteil/libcgi,rafaelsteil/libcgi,LeSpocky/libcgi