move VFP_Push() declaration

it is part of the filter side of things, while the remaining
declarations in cache_varnishd concern the interface between filters and
the rest of the code.

This also parallels the location of VDP_Push()

bin/varnishd/cache/cache_filter.h

@@ -87,6 +87,7 @@ struct vfp_ctx {
 	unsigned		obj_flags;
 };
 
+struct vfp_entry *VFP_Push(struct vfp_ctx *, const struct vfp *);
 enum vfp_status VFP_Suck(struct vfp_ctx *, void *p, ssize_t *lp);
 enum vfp_status VFP_Error(struct vfp_ctx *, const char *fmt, ...)
     v_printflike_(2, 3);

bin/varnishd/cache/cache_varnishd.h

@@ -248,7 +248,6 @@ void Bereq_Rollback(struct busyobj *);
 void VFP_Init(void);
 enum vfp_status VFP_GetStorage(struct vfp_ctx *, ssize_t *sz, uint8_t **ptr);
 void VFP_Extend(const struct vfp_ctx *, ssize_t sz);
-struct vfp_entry *VFP_Push(struct vfp_ctx *, const struct vfp *);
 void VFP_Setup(struct vfp_ctx *vc, struct worker *wrk);
 int VFP_Open(struct vfp_ctx *bo);
 void VFP_Close(struct vfp_ctx *bo);