Add xwalk_core_library to git ignore path
Add xwalk_core_library to git ignore path

Otherwise, buildbot will not pack it into tarball.

hgl888/crosswalk-cordova-android,crosswalk-project/crosswalk-cordova-android,chengxiaole/crosswalk-cordova-android,hgl888/crosswalk-cordova-android,awesome-niu/crosswalk-cordova-android,rakuco/crosswalk-cordova-android,rakuco/crosswalk-cordova-android,hgl888/crosswalk-cordova-android,awesome-niu/crosswalk-cordova-android,awesome-niu/crosswalk-cordova-android,chengxiaole/crosswalk-cordova-android,crosswalk-project/crosswalk-cordova-android,rakuco/crosswalk-cordova-android,chengxiaole/crosswalk-cordova-android,crosswalk-project/crosswalk-cordova-android