forward declare struct timespec

... like we do so for struct timeval at several hundreds of lines above.
Depending on OS/Compiler, this can be the first place for the struct to
appear.  To make sure the struct is global, we need a forward
declaration at this point.

include/ruby/intern.h

@@ -952,6 +952,7 @@ VALUE rb_mutex_unlock(VALUE mutex);
 VALUE rb_mutex_sleep(VALUE self, VALUE timeout);
 VALUE rb_mutex_synchronize(VALUE mutex, VALUE (*func)(VALUE arg), VALUE arg);
 /* time.c */
+struct timespec;
 void rb_timespec_now(struct timespec *);
 VALUE rb_time_new(time_t, long);
 VALUE rb_time_nano_new(time_t, long);