Add ignored recastnavigation files to .gitigore

.gitignore

@@ -82,3 +82,13 @@ moc_*.cxx
 *.[ao]
 *.so
 venv/
+
+## recastnavigation unused files
+extern/recastnavigation/.travis.yml
+extern/recastnavigation/CONTRIBUTING.md
+extern/recastnavigation/Docs/
+extern/recastnavigation/Doxyfile
+extern/recastnavigation/README.md
+extern/recastnavigation/RecastDemo/
+extern/recastnavigation/Tests/
+extern/recastnavigation/appveyor.yml