fixed a define around a unistd include

This commit was SVN r5807.

src/communicator/comm_dyn.c

@@ -20,7 +20,9 @@
 #ifdef HAVE_SYS_UIO_H
 #include <sys/uio.h>
 #endif
+#ifdef HAVE_UNISTD_H
 #include <unistd.h>
+#endif
 
 #include "mpi.h"
 #include "dps/dps.h"