Fix "make check"; manually include <string.h> since the datatype

header files were re-orged to have fewer dependencies

This commit was SVN r17427.

test/datatype/position.c

@@ -11,6 +11,7 @@
  */
 
 #include "ompi_config.h"
+#include <string.h>
 #include "ompi/datatype/convertor.h"
 #include "ompi/datatype/datatype.h"
 #include "opal/util/output.h"