rtpdec: make the NTP time values unsigned.

As per RFC3550, section 4, the NTP time is provided as 64-bit unsigned
integer, so follow the same logic here.
Reviewed-by: Luca Barbato <lu_zero@gentoo.org>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavformat/rtpdec.h

@@ -173,9 +173,9 @@ struct RTPDemuxContext {
     /*@}*/
 
     /* rtcp sender statistics receive */
-    int64_t last_rtcp_ntp_time;
+    uint64_t last_rtcp_ntp_time;
     int64_t last_rtcp_reception_time;
-    int64_t first_rtcp_ntp_time;
+    uint64_t first_rtcp_ntp_time;
     uint32_t last_rtcp_timestamp;
     int64_t rtcp_ts_offset;