xmlsecurity: use auto when declaring iterators

Change-Id: Ie56c0c0e1917f159957babca346f1a3fa04bc629
Reviewed-on: https://gerrit.libreoffice.org/35253
Reviewed-by: Miklos Vajna <vmiklos@collabora.co.uk>
Tested-by: Jenkins <ci@libreoffice.org>

xmlsecurity/source/pdfio/pdfdocument.cxx

@@ -1339,8 +1339,8 @@ size_t PDFDocument::FindStartXRef(SvStream& rStream)
         aBuf.resize(nSize);
     OString aPrefix("startxref");
     // Find the last startxref at the end of the document.
-    std::vector<char>::iterator itLastValid = aBuf.end();
-    std::vector<char>::iterator it = aBuf.begin();
+    auto itLastValid = aBuf.end();
+    auto it = aBuf.begin();
     while (true)
     {
         it = std::search(it, aBuf.end(), aPrefix.getStr(), aPrefix.getStr() + aPrefix.getLength());