sys/epoll.h: add EPOLLNVAL from linux v4.16

added to uapi in commit 65aaf87b3aa2d049c6b9fd85221858a895df3393
used since commit a9a08845e9acbd224e4ee466f5c1275ed50054e8,
which renamed POLL* to EPOLL* in the kernel.

include/sys/epoll.h

@@ -21,6 +21,7 @@ enum EPOLL_EVENTS { __EPOLL_DUMMY };
 #define EPOLLPRI 0x002
 #define EPOLLOUT 0x004
 #define EPOLLRDNORM 0x040
+#define EPOLLNVAL 0x020
 #define EPOLLRDBAND 0x080
 #define EPOLLWRNORM 0x100
 #define EPOLLWRBAND 0x200