mohqueue: fix build warning

> CC (gcc) [M mohqueue.so]		mohq.o
> mohq.c: In function 'mod_init':
> mohq.c:452:25: warning: passing argument 1 of 'pv_cache_get' from incompatible pointer type
>  prtp_pv = pv_cache_get (&prtpstat);
>                         ^
> In file included from ../rr/../../sr_module.h:44:0,
>                 from ../rr/api.h:43,
>                 from mohq_common.h:28,
>                 from mohq.h:25,
>                 from mohq.c:25:
> ../rr/../../pvar.h:208:12: note: expected 'struct str *' but argument is of type 'struct str (*)[1]'
>  pv_spec_t* pv_cache_get(str *name);
>            ^
> CC (gcc) [M mohqueue.so]		mohq_locks.o

modules/mohqueue/mohq.c

@@ -449,7 +449,7 @@ if(rtplen != prtpstat->len)
   LM_ERR ("Unable to find RTPSTAT pv!\n");
   goto initerr;
   }
-prtp_pv = pv_cache_get (&prtpstat);
+prtp_pv = pv_cache_get (&prtpstat[0]);
 if(!prtp_pv)
   {
   LM_ERR ("Unable to find pv spec for RTPSTAT!\n");