[MINOR] use 'is_set' instead of 'isset' in struct poller
'isset' was defined as a macro in /usr/include/sys/param.h, and
it breaks build on at least OpenBSD.
diff --git a/include/proto/fd.h b/include/proto/fd.h
index be52612..9b4c632 100644
--- a/include/proto/fd.h
+++ b/include/proto/fd.h
@@ -53,7 +53,7 @@
#define EV_FD_SET(fd, ev) (cur_poller.set((fd), (ev)))
#define EV_FD_CLR(fd, ev) (cur_poller.clr((fd), (ev)))
-#define EV_FD_ISSET(fd, ev) (cur_poller.isset((fd), (ev)))
+#define EV_FD_ISSET(fd, ev) (cur_poller.is_set((fd), (ev)))
#define EV_FD_COND_S(fd, ev) (cur_poller.cond_s((fd), (ev)))
#define EV_FD_COND_C(fd, ev) (cur_poller.cond_c((fd), (ev)))
#define EV_FD_REM(fd) (cur_poller.rem(fd))