Fix TAO_PEGTL_NAMESPACE for 3.x

[wravery]

No description provided.

[codecov-commenter]

Codecov Report

Merging #317 (9418e0e) into 3.x (0ef2d22) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##              3.x     #317   +/-   ##
=======================================
  Coverage   99.70%   99.70%           
=======================================
  Files         247      247           
  Lines        5042     5042           
=======================================
  Hits         5027     5027           
  Misses         15       15           

Impacted Files	Coverage Δ
include/tao/pegtl/demangle.hpp	100.00% <ø> (ø)

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[d-frey]

Thanks!