timers.h: add declaration for timeval.

This frees us from some dependencies on include order.

src/lib/evloop/timers.h

@@ -13,6 +13,7 @@
 #include "lib/testsupport/testsupport.h"
 
 struct monotime_t;
+struct timeval;
 typedef struct timeout tor_timer_t;
 typedef void (*timer_cb_fn_t)(tor_timer_t *, void *,
                               const struct monotime_t *);