fix typo when using sizeof() to determine the size of the RTCP header

This was spotted by zecke, thanks.
diff --git a/openbsc/src/rtp_proxy.c b/openbsc/src/rtp_proxy.c
index 61fc385..158145b 100644
--- a/openbsc/src/rtp_proxy.c
+++ b/openbsc/src/rtp_proxy.c
@@ -68,7 +68,7 @@
 
 	rtcp_end = cur + *rtcp_len;
 	/* move cur to end of RTP header */
-	cur += sizeof(rh);
+	cur += sizeof(*rh);
 
 	/* iterate over Chunks */
 	while (cur+4 < rtcp_end) {