Fix forward declaration typo in UlpfecGenerator.

BUG=webrtc:5654
NOTRY=true

Review-Url: https://codereview.webrtc.org/2481343004
Cr-Commit-Position: refs/heads/master@{#14963}
diff --git a/webrtc/modules/rtp_rtcp/source/ulpfec_generator.h b/webrtc/modules/rtp_rtcp/source/ulpfec_generator.h
index faff58f..52e03cd 100644
--- a/webrtc/modules/rtp_rtcp/source/ulpfec_generator.h
+++ b/webrtc/modules/rtp_rtcp/source/ulpfec_generator.h
@@ -19,7 +19,7 @@
 
 namespace webrtc {
 
-class FlexfecSenderImpl;
+class FlexfecSender;
 
 class RedPacket {
  public: