.gitignore: add .nb-gradle folder to ignore list

.gitignore

@@ -132,3 +132,4 @@
 !/jme3-bullet-native/libs/native/osx/x86_64/libbulletjme.dylib
 !/jme3-bullet-native/libs/native/linux/x86/libbulletjme.so
 !/jme3-bullet-native/libs/native/linux/x86_64/libbulletjme.so
+/.nb-gradle/