Include <sys/poll.h> only if there is no <poll.h>

* pathtrace.c: Do not include <sys/poll.h> if <poll.h> is available.
* stream.c: Likewise.
diff --git a/stream.c b/stream.c
index 18a8ca9..f0c2af4 100644
--- a/stream.c
+++ b/stream.c
@@ -27,10 +27,9 @@
  */
 
 #include "defs.h"
-#ifdef HAVE_POLL_H
+#if defined HAVE_POLL_H
 # include <poll.h>
-#endif
-#ifdef HAVE_SYS_POLL_H
+#elif defined HAVE_SYS_POLL_H
 # include <sys/poll.h>
 #endif
 #ifdef HAVE_SYS_CONF_H