Fix range-loop-construct warnings

[rst0git]

This pull request fixes the following warnings reported by g++ (GCC) 11:

P4Objects.cpp:656:21: warning: loop variable 'cfg_field' creates a copy from type 'const Json::Value' [-Wrange-loop-construct]
  656 |     for (const auto cfg_field : cfg_fields) {
      |                     ^~~~~~~~~
P4Objects.cpp:656:21: note: use reference type to prevent copying
  656 |     for (const auto cfg_field : cfg_fields) {
      |                     ^~~~~~~~~
      |                     &

simple_pre.cpp:187:19: warning: loop variable 'p' creates a copy from type 'const std::pair<const long unsigned int, bm::McSimplePre::MgidEntry>' [-Wrange-loop-construct]
  187 |   for (const auto p : mgid_entries) {
      |                   ^
simple_pre.cpp:187:19: note: use reference type to prevent copying
  187 |   for (const auto p : mgid_entries) {
      |                   ^
      |                   &

simple_pre.cpp:215:19: warning: loop variable 'p' creates a copy from type 'const std::pair<const long unsigned int, bm::McSimplePre::L2Entry>' [-Wrange-loop-construct]
  215 |   for (const auto p : l2_entries) {
      |                   ^
simple_pre.cpp:215:19: note: use reference type to prevent copying
  215 |   for (const auto p : l2_entries) {
      |                   ^
      |                   &

[antoninbas]

Build failure seems unrelated. I'll open a separate PR for it.