Generator: Add missing std namespace to string arguments for Protobuf 3.20 compatibility

[traversaro]

🦟 Bug fix

Fixes the following error that occurs when building the library against Protobuf 3.20 :

[  0%] Building CXX object src/CMakeFiles/ign_msgs_gen.dir/Generator.cc.o
In file included from /home/conda/feedstock_root/build_artifacts/libignition-msgs5_1649622632208/work/src/Generator.cc:40:
/home/conda/feedstock_root/build_artifacts/libignition-msgs5_1649622632208/work/src/Generator.hh:47:21: error: 'string' does not name a type; did you mean 'strings'?
   47 |               const string &_parameter,
      |                     ^~~~~~
      |                     strings
/home/conda/feedstock_root/build_artifacts/libignition-msgs5_1649622632208/work/src/Generator.hh:49:15: error: 'string' has not been declared
   49 |               string *_error) const;
      |               ^~~~~~
/home/conda/feedstock_root/build_artifacts/libignition-msgs5_1649622632208/work/src/Generator.cc:70:38: error: 'string' does not name a type; did you mean 'strings'?
   70 |                                const string &/*_parameter*/,
      |                                      ^~~~~~
      |                                      strings

This was detected in conda-forge/libignition-msgs1-feedstock#73 .

Summary

Several uses of string were missing std::, probably with protobuf<3.20 everything was working just by chance.

Checklist

• Signed all commits for DCO
• Added tests
• Updated documentation (as needed)
• Updated migration guide (as needed)
• codecheck passed (See contributing)
• All tests passed (See test coverage)
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[codecov]

Codecov Report

Merging #242 (0ab1146) into ign-msgs5 (d11fe38) will not change coverage.
The diff coverage is n/a.

❗ Current head 0ab1146 differs from pull request most recent head 0e87025. Consider uploading reports for the commit 0e87025 to get more accurate results

@@            Coverage Diff             @@
##           ign-msgs5     #242   +/-   ##
==========================================
  Coverage      85.27%   85.27%           
==========================================
  Files              9        9           
  Lines            903      903           
==========================================
  Hits             770      770           
  Misses           133      133           

Impacted Files	Coverage Δ
src/Generator.cc	92.30% <ø> (ø)

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