adding tinyxml2 as subrepo

Former-commit-id: ce7d93c90ae65ef497461ca514fef9396d03822d

.gitmodules

@@ -15,3 +15,6 @@ url=https://github.com/libigl/nanogui.git
 [submodule "external/triangle"]
 	path = external/triangle
 	url = https://github.com/libigl/triangle.git
+[submodule "external/tinyxml2"]
+	path = external/tinyxml2
+	url = https://github.com/leethomason/tinyxml2.git