add linux extension POLLRDHUP to poll.h

the POLL prefix is in the reserved namespace for poll.h, so no feature
test macro checks are needed.

include/poll.h

@@ -18,6 +18,7 @@ extern "C" {
 #define POLLWRNORM 0x100
 #define POLLWRBAND 0x200
 #define POLLMSG    0x400
+#define POLLRDHUP  0x2000
 
 typedef unsigned long nfds_t;