nviz_cmd: add missing header

git-svn-id: https://svn.osgeo.org/grass/grass/trunk@47068 15284696-431f-4ddb-bdfa-cd5b030d7da7

visualization/nviz2/cmd/volume.c

@@ -13,6 +13,7 @@
 */
 
 #include <stdlib.h>
+#include <string.h>
 
 #include <grass/G3d.h>
 #include <grass/glocale.h>