tt/mini_isam: use proper include, get rid of ultrix strdup

cde/lib/tt/mini_isam/isalloc.c

@@ -42,13 +42,7 @@ static char sccsid[] = "@(#)isalloc.c 1.3 89/07/17 Copyr 1988 Sun Micro";
 
 #include "isam_impl.h"
 #include <stdlib.h>
-
-/* strdup is not defined in Ultrix' string.h, so it is implemented in a separate
- * file named strdup.ultrix.c
- */
-#ifdef __ultrix__
-extern char *strdup();
-#endif
+#include <string.h>
 
 /*
  * _ismalloc(nbytes)