Include CertificateSigning as allowed KeyUsage for the self-signed cert

The certificate verification in open62541 1.4 checks for a self-signed certificate
by calling X509_check_issued(). This fails with X509_V_ERR_KEYUSAGE_NO_CERTSIGN
if a self-signed certificate without CertificateSigning is encountered.

src/x509certificate.cpp

@@ -82,6 +82,7 @@ bool X509Certificate::createCertificate(const QString &pkiDir)
     ku->setKeyUsage(QOpcUaX509ExtensionKeyUsage::KeyUsage::NonRepudiation);
     ku->setKeyUsage(QOpcUaX509ExtensionKeyUsage::KeyUsage::KeyEncipherment);
     ku->setKeyUsage(QOpcUaX509ExtensionKeyUsage::KeyUsage::DataEncipherment);
+    ku->setKeyUsage(QOpcUaX509ExtensionKeyUsage::KeyUsage::CertificateSigning);
     csr.addExtension(ku);
 
     // Set the extended key usage constraints