Hook up new "rtc_enable_sctp" build argument to "HAVE_SCTP" define.

This allows building without SCTP support (and even building/running
tests). The "HAVE_SCTP" define has been functional for a while, but there
wasn't any easy way to turn it on/off.

NOTRY=True
BUG=webrtc:6933

Review-Url: https://codereview.webrtc.org/2593313002
Cr-Commit-Position: refs/heads/master@{#15763}
diff --git a/webrtc/api/rtcstats_integrationtest.cc b/webrtc/api/rtcstats_integrationtest.cc
index c28130fe..d78ba35 100644
--- a/webrtc/api/rtcstats_integrationtest.cc
+++ b/webrtc/api/rtcstats_integrationtest.cc
@@ -523,6 +523,7 @@
   rtc::scoped_refptr<const RTCStatsReport> report_;
 };
 
+#ifdef HAVE_SCTP
 TEST_F(RTCStatsIntegrationTest, GetStatsFromCaller) {
   StartCall();
 
@@ -549,6 +550,7 @@
   // the peer connection.
   EXPECT_TRUE(stats_obtainer->report());
 }
+#endif  // HAVE_SCTP
 
 }  // namespace