mark RTPHeader struct RTC_EXPORT

so it can be used in downstream Chrome tests

BUG=None

Change-Id: I4b3e1f172e8eb2ba01ab5c257f3626223781da31
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/291116
Reviewed-by: Harald Alvestrand <hta@webrtc.org>
Reviewed-by: Florent Castelli <orphis@webrtc.org>
Commit-Queue: Philipp Hancke <phancke@microsoft.com>
Cr-Commit-Position: refs/heads/main@{#39163}
diff --git a/api/rtp_headers.h b/api/rtp_headers.h
index a640eb7..743fd6d 100644
--- a/api/rtp_headers.h
+++ b/api/rtp_headers.h
@@ -148,7 +148,7 @@
 
 enum { kRtpCsrcSize = 15 };  // RFC 3550 page 13
 
-struct RTPHeader {
+struct RTC_EXPORT RTPHeader {
   RTPHeader();
   RTPHeader(const RTPHeader& other);
   RTPHeader& operator=(const RTPHeader& other);