[wrt][XWALK-2792]fix wgt signing bug

[Detailed commit message (if have)]
Impacted TCs num(designed): New 0, update 0, deleted 0
Unit test platform: [tizen]
The test app can be installed/uninstalled.
Merge pull request #1080 from linshen/shenlin/updatewgt-xwalk2792

[wrt][XWALK-2792]fix wgt signing bug