Fixed --test-build-eventport on macOS 10.12 and later.

In macOS 10.12, CLOCK_REALTIME and clockid_t were added, but not timer_t.
diff --git a/src/event/modules/ngx_eventport_module.c b/src/event/modules/ngx_eventport_module.c
index e723f92..01cfc97 100644
--- a/src/event/modules/ngx_eventport_module.c
+++ b/src/event/modules/ngx_eventport_module.c
@@ -19,6 +19,8 @@
 #define CLOCK_REALTIME          0
 typedef int     clockid_t;
 typedef void *  timer_t;
+#elif (NGX_DARWIN)
+typedef void *  timer_t;
 #endif
 
 /* Solaris declarations */