[Tizen] Check signature value using xmlsec1 library

Based on API provided by xmlsec1 library, signature value and reference are checked.
It will report below error:
../../xwalk/application/common/tizen/signature_xmlsec_adaptor.cc: In static member function ‘static _xmlSecKeysMngr* {anonymous}::XmlSecContext::LoadTrustedCerts(const xwalk::application::SignatureData&)’:
../../xwalk/application/common/tizen/signature_xmlsec_adaptor.cc:170:99: error: invalid static_cast from type ‘const char*’ to type ‘const unsigned char*’
     if (xmlSecCryptoAppKeysMngrCertLoadMemory(mngr, static_cast<const unsigned char*>(cert.c_str()), cert.size(),