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/pc/BUILD.gn b/webrtc/pc/BUILD.gn
index fe8711f..f2bdbc3 100644
--- a/webrtc/pc/BUILD.gn
+++ b/webrtc/pc/BUILD.gn
@@ -15,10 +15,10 @@
 }
 
 config("rtc_pc_config") {
-  defines = [
-    "HAVE_SCTP",
-    "HAVE_SRTP",
-  ]
+  defines = [ "HAVE_SRTP" ]
+  if (rtc_enable_sctp) {
+    defines += [ "HAVE_SCTP" ]
+  }
 }
 
 rtc_static_library("rtc_pc") {