Fix sqlb::Table::fieldList()

This suddenly failed on my system and broke the CSV import. Maybe
because of some new version of some software? Either way, the new
version should work anywhere.

src/sqlitetypes.cpp

@@ -146,7 +146,7 @@ QStringList Table::fieldList() const
 {
     QStringList sl;
 
-    foreach(FieldPtr f, m_fields) {
+    foreach(const FieldPtr& f, m_fields) {
         sl << f->toString();
     }