Added missing includes core.scad which generates warnings with latest OpenSCAD snapshots.

Note, however, NopSCADlib does not work with the current OpenSCAD snapshot.

tests/hygrometer.scad

@@ -16,6 +16,7 @@
 // You should have received a copy of the GNU General Public License along with NopSCADlib.
 // If not, see <https://www.gnu.org/licenses/>.
 //
+include <../core.scad>
 use <../vitamins/hygrometer.scad>
 
 if($preview)