refactor: source files have no header guards

[alandefreitas]

fix #819

[cppalliance-bot]

GCOVR code coverage report https://824.url.prtest2.cppalliance.org/gcovr/index.html
LCOV code coverage report https://824.url.prtest2.cppalliance.org/genhtml/index.html
Coverage Diff https://824.url.prtest2.cppalliance.org/gcovr/coverage_diff.txt

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 97.24%. Comparing base (0c16da3) to head (406a078).

Additional details and impacted files

@@           Coverage Diff            @@
##           develop     #824   +/-   ##
========================================
  Coverage    97.24%   97.24%           
========================================
  Files          157      157           
  Lines         8595     8595           
========================================
  Hits          8358     8358           
  Misses         237      237           

Files	Coverage Δ
src/authority_view.cpp	85.11% <ø> (ø)
src/decode_view.cpp	100.00% <ø> (ø)
src/detail/any_params_iter.cpp	99.44% <ø> (ø)
src/detail/any_segments_iter.cpp	100.00% <ø> (ø)
src/detail/decode.cpp	85.96% <ø> (ø)
src/detail/except.cpp	100.00% <ø> (ø)
src/detail/format_args.cpp	100.00% <ø> (ø)
src/detail/normalize.cpp	91.35% <ø> (ø)
src/detail/params_iter_impl.cpp	97.58% <ø> (ø)
src/detail/pattern.cpp	100.00% <ø> (ø)
... and 56 more

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