add missing header

src/silo/silo_ns.c

@@ -49,6 +49,7 @@ reflect those  of the United  States Government or  Lawrence Livermore
 National  Security, LLC,  and shall  not  be used  for advertising  or
 product endorsement purposes.
 */
+#include <ctype.h>
 #include <stdarg.h>
 #include <stdlib.h>
 #include <errno.h>