Add missing extension for pomless build

Signed-off-by: Alexander Diewald <diewald@fortiss.org>

.mvn/extensions.xml

+<?xml version="1.0" encoding="UTF-8"?>
+<extensions>
+  <extension>
+    <groupId>org.eclipse.tycho.extras</groupId>
+    <artifactId>tycho-pomless</artifactId>
+    <version>1.3.0</version>
+  </extension>
+</extensions>