backport of https://github.com/apple/swift-corelibs-libdispatch/pull/559

Index: src/event/event_internal.h
--- src/event/event_internal.h.orig
+++ src/event/event_internal.h
@@ -125,6 +125,8 @@ _dispatch_timer_flags_from_clock(dispatch_clock_t cloc
 
 #if defined(_WIN32)
 typedef uintptr_t dispatch_unote_ident_t;
+#elif defined(__OpenBSD__)
+typedef uintptr_t dispatch_unote_ident_t;
 #else
 typedef uint32_t dispatch_unote_ident_t;
 #endif