Add missing #include <stdlib.h> to display/d.where/main.c. (#1377)

It is requiref for `exit()` and `EXIT_SUCCESS`/`EXIT_FAILURE`.

display/d.where/main.c

@@ -19,6 +19,7 @@
  *               for details.
  *
  *****************************************************************************/
+#include <stdlib.h>
 #include <string.h>
 #include <grass/gis.h>
 #include <grass/gprojects.h>