enable inbound-late-negotiation in sample configs

We often tell people to enable this for various reasons; it likely
makes a sensible default.

conf/sbc/sbc_profiles/external.xml

@@ -48,6 +48,7 @@
     <param name="nonce-ttl" value="60"/>
     <param name="auth-calls" value="false"/>
     <param name="rtp-timeout-sec" value="1800"/>
+    <param name="inbound-late-negotiation" value="true"/>
     <!--
         DO NOT USE HOSTNAMES, ONLY IP ADDRESSES IN THESE SETTINGS!
     -->