goto-cc: Fix compilation with GCC 11

[vmihalko]

The reference type caused following error:
/builddir/build/BUILD/cbmc/src/goto-cc/ms_link_cmdline.cpp:337:26: error: loop variable 'ms_link_option' of type 'const string&' {aka 'const std::__cxx11::basic_string&'} binds to a temporary constructed from type 'const char*' [-Werror=range-loop-construct]
337 | for(const std::string &ms_link_option : ms_link_options)
| ^~~~~~~~~~~~~~
/builddir/build/BUILD/cbmc/src/goto-cc/ms_link_cmdline.cpp:337:26: note: use non-reference type 'const string' {aka 'const std::__cxx11::basic_string'} to make the copy explicit or 'const char* const&' to prevent copying

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• Methods or procedures I have added are documented, following the guidelines provided in CODING_STANDARD.md.
• The feature or user visible behaviour I have added or modified has been documented in the User Guide in doc/cprover-manual/
• Regression or unit tests are included, or existing tests cover the modified code (in this case I have detailed which ones those are in the commit message).
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[codecov]

Codecov Report

Merging #6032 (9db8e6c) into develop (eea16dd) will increase coverage by 0.01%.
The diff coverage is 0.00%.

@@             Coverage Diff             @@
##           develop    #6032      +/-   ##
===========================================
+ Coverage    74.11%   74.12%   +0.01%     
===========================================
  Files         1444     1444              
  Lines       157389   157389              
===========================================
+ Hits        116646   116664      +18     
+ Misses       40743    40725      -18     

Impacted Files	Coverage Δ
src/goto-cc/ms_link_cmdline.cpp	0.00% <0.00%> (ø)
src/ansi-c/parser.y	61.00% <0.00%> (+0.06%)	⬆️
src/ansi-c/c_typecheck_expr.cpp	60.09% <0.00%> (+0.29%)	⬆️
src/ansi-c/c_typecheck_code.cpp	70.14% <0.00%> (+0.72%)	⬆️
src/ansi-c/literals/convert_character_literal.cpp	51.21% <0.00%> (+19.51%)	⬆️

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