clang-tidy: remove const from data members

[neheb]

Found with cppcoreguidelines-avoid-const-or-ref-data-members

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[codecov]

Codecov Report

Merging #2572 (1281818) into main (0fa22ed) will decrease coverage by 0.02%.
The diff coverage is 42.85%.

@@            Coverage Diff             @@
##             main    #2572      +/-   ##
==========================================
- Coverage   63.90%   63.89%   -0.02%     
==========================================
  Files         103      103              
  Lines       22393    22397       +4     
  Branches    10824    10826       +2     
==========================================
  Hits        14310    14310              
- Misses       5864     5867       +3     
- Partials     2219     2220       +1     

Impacted Files	Coverage Δ
include/exiv2/bmffimage.hpp	100.00% <ø> (ø)
include/exiv2/error.hpp	54.54% <ø> (ø)
src/basicio.cpp	50.58% <ø> (ø)
src/cr2header_int.hpp	0.00% <ø> (ø)
src/image_int.hpp	100.00% <ø> (ø)
src/makernote_int.cpp	65.85% <0.00%> (-0.50%)	⬇️
src/nikonmn_int.cpp	61.09% <ø> (ø)
src/sonymn_int.cpp	79.96% <ø> (ø)
src/tiffcomposite_int.hpp	97.27% <ø> (ø)
src/tiffimage_int.hpp	100.00% <ø> (ø)
... and 3 more

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