ignore dirty in nanogui

Former-commit-id: c8f71f0e4dbc81473cdbf1ab2000d1e630687761

.gitmodules

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