Fix#2223

[b-scholz]

The C++ generated code of a Souffle program with functors is put into the namespace souffle::functors to avoid name clashes with the Souffle system. No changes are needed in the functor libraries because the function names are defined as external "C" functions, and hence their symbols do not carry their namespace.
https://stackoverflow.com/questions/28996944/extern-c-linkage-inside-c-namespace

[codecov]

Codecov Report

Merging #2224 (6eb3ad0) into master (f16d18d) will decrease coverage by 0.26%.
The diff coverage is 67.89%.

@@            Coverage Diff             @@
##           master    #2224      +/-   ##
==========================================
- Coverage   75.52%   75.25%   -0.27%     
==========================================
  Files         452      454       +2     
  Lines       27479    27572      +93     
==========================================
- Hits        20753    20750       -3     
- Misses       6726     6822      +96     

Impacted Files	Coverage Δ
src/RelationTag.h	52.23% <0.00%> (-0.80%)	⬇️
src/ast/ExecutionOrder.h	100.00% <ø> (ø)
src/ast/Program.h	100.00% <ø> (ø)
src/ast/transform/ComponentInstantiation.cpp	92.36% <ø> (+0.38%)	⬆️
src/ast/transform/MinimiseProgram.h	100.00% <ø> (ø)
src/ast/transform/ResolveAliases.cpp	98.93% <ø> (ø)
src/ast/utility/SipsMetric.h	18.18% <ø> (ø)
src/ast/utility/Utils.h	100.00% <ø> (ø)
src/ast/utility/Visitor.h	57.33% <0.00%> (ø)
src/ast2ram/provenance/ClauseTranslator.h	100.00% <ø> (ø)
... and 133 more

[b-scholz]

This is related to issue #2223.