Add missing <string.h>

This commit was SVN r5046.

src/dps/dps_internal_functions.c

@@ -22,6 +22,7 @@
 #include <sys/types.h>
 #include <math.h>
 #include <unistd.h>
+#include <string.h>
 
 #include "util/output.h"