Address warnings from g++-12

Author: eddelbuettel

src/attributes.cpp

@@ -1621,7 +1621,7 @@ namespace attributes {
         // Look for the signature termination ({ or ; not inside quotes)
         // on this line and then subsequent lines if necessary
         std::string signature;
-        for (size_t i = lineNumber; i<lines_.size(); i++) {
+        for (size_t i = lineNumber; i < (size_t)lines_.size(); i++) {
             std::string line;
             line = lines_[i];
             bool insideQuotes = false;