Fix implicit decalaration in rb_event_thread.c

src/ruby/ext/grpc/rb_event_thread.c

@@ -29,6 +29,8 @@
 #include <grpc/support/time.h>
 #include <ruby/thread.h>
 
+#include "rb_grpc.h"
+
 typedef struct grpc_rb_event {
   // callback will be called with argument while holding the GVL
   void (*callback)(void*);