clang-tidy: replace + with append()

Silences clang-tidy performance warning

Signed-off-by: Rosen Penev <rosenp@gmail.com>

src/nikonmn_int.cpp

@@ -3233,9 +3233,9 @@ std::ostream& Nikon3MakerNote::print0x009e(std::ostream& os, const Value& value,
     std::string d = s.empty() ? "" : "; ";
     const TagDetails* td = find(nikonRetouchHistory, l);
     if (td) {
-      s = std::string(exvGettext(td->label_)) + d + s;
+      s = std::string(exvGettext(td->label_)).append(d).append(s);
     } else {
-      s = std::string(_("Unknown")) + std::string(" (") + toString(l) + std::string(")") + d + s;
+      s = std::string(_("Unknown")).append(" (").append(toString(l)).append(")").append(d).append(s);
     }
   }
   return os << s;