MT#55283 fix compilation for older kernels

Older kernels (3.x) have ktime_t defined as a union, which means we
can't just treat it as a number and must wrap it inside {}. This still
also works for later kernels which have it typedef'd as an int.

closes #1708

Change-Id: I6195e45eb80f6e430e97247f62cd5f6696450b09

kernel-module/xt_RTPENGINE.c

@@ -5450,7 +5450,7 @@ static unsigned int rtpengine46(struct sk_buff *skb, struct sk_buff *oskb,
 		skb_trim(skb, rtp.header_len + rtp.payload_len);
 		if (is_rtcp == RTCP_FORWARD) {
 			// mark packet as "handled" with negative timestamp
-			oskb->tstamp = -oskb->tstamp;
+			oskb->tstamp = (ktime_t) {-ktime_to_ns(oskb->tstamp)};
 			nf_action = XT_CONTINUE;
 		}
 	}