give opentracker the missing header file

opentracker.c

@@ -18,6 +18,7 @@
 #include <sys/types.h>
 #include <sys/stat.h>
 #include <sys/socket.h>
+#include <sys/mman.h>
 #include <arpa/inet.h>
 #include <unistd.h>
 #include <stdlib.h>