Include <errno.h> in engine_poll.c for EINTR.

I'm not sure why I wasn't seeing this error before:
engine_poll.c: In function ‘poll_loop’:
engine_poll.c:352:46: error: ‘EINTR’ undeclared (first use in this function)
engine_poll.c:352:46: note: each undeclared identifier is reported only once for each function it appears in

nsock/src/engine_poll.c

@@ -59,6 +59,8 @@
 #define _GNU_SOURCE
 #endif
 
+#include <errno.h>
+
 #ifdef HAVE_CONFIG_H
 #include "nsock_config.h"
 #elif WIN32