Added Silan to list of third party code used by Airtime

LICENSE_3RD_PARTY

@@ -141,6 +141,12 @@ Non-linked code:
    - Web site: http://www.python.org/
    - License: PSF License. See http://docs.python.org/license.html
 
+ * Silan
+   - What is it: Silence detector
+   - Web site: https://github.com/x42/silan
+   - License: GPL-2+
+   - Compatible with GPLv3? Yes.
+
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