make EXP_Rearm() non-private again - vmods use it

Lack of this interface breaks existing vmods, so should we really need to
remove it, we problable should provide an alternative.

bin/varnishd/cache/cache.h

@@ -713,6 +713,10 @@ extern pthread_t cli_thread;
 #define EXP_WHEN(to)							\
 	((to)->t_origin + (to)->ttl + (to)->grace + (to)->keep)
 
+/* cache_exp.c */
+void EXP_Rearm(struct objcore *, double now, double ttl, double grace,
+    double keep);
+
 /* cache_fetch.c */
 enum vbf_fetch_mode_e {
 	VBF_NORMAL = 0,

bin/varnishd/cache/cache_priv.h

@@ -45,8 +45,6 @@ void VBP_Init(void);
 /* cache_exp.c */
 double EXP_Ttl(const struct req *, const struct objcore *);
 void EXP_Insert(struct worker *wrk, struct objcore *oc);
-void EXP_Rearm(struct objcore *, double now, double ttl, double grace,
-    double keep);
 void EXP_Remove(struct objcore *);
 
 /* From cache_main.c */