Disable quic by default; enable with build tag

Taking a more extreme approach than #588... This patch
disables quic by default. The advantage here would be
that webrtc builds with older versions of go than
quic-go supports (currently 1.12+).

examples/ortc-quic/main.go

@@ -1,3 +1,5 @@
+// +build quic
+
 package main
 
 import (