Add 3 missing RTC_EXPORT.

These two annotations are now needed to correctly compile Chromium
with is_component_build=true and the WebRTC component.

TBR: kwiberg@webrtc.org
Bug: webrtc:9419
Change-Id: Id5603cf747357c0c2a4b41684eb4fd607cccfdea
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/158881
Commit-Queue: Mirko Bonadei <mbonadei@webrtc.org>
Reviewed-by: Karl Wiberg <kwiberg@webrtc.org>
Reviewed-by: Mirko Bonadei <mbonadei@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#29686}

api/jsep.h

diff --git a/api/jsep.h b/api/jsep.h
index 7b4934a..86f4162 100644
--- a/api/jsep.h
+++ b/api/jsep.h
@@ -112,7 +112,7 @@
 
 // Returns the string form of the given SDP type. String forms are defined in
 // SessionDescriptionInterface.
-const char* SdpTypeToString(SdpType type);
+RTC_EXPORT const char* SdpTypeToString(SdpType type);
 
 // Returns the SdpType from its string form. The string form can be one of the
 // constants defined in SessionDescriptionInterface. Passing in any other string