ignore dirty in nanogui

Former-commit-id: 38079cc79eca808f97259b5a174510855b8989fe

.gitmodules

@@ -2,3 +2,5 @@
 	path = external/nanogui
 	url = https://github.com/schuellc/nanogui.git
         fetchRecursiveSubmodules = true
+        ignore = dirty
+