Make FindInih also make finding reader required (backport #2721)

[mergify]

This is an automatic backport of pull request #2721 done by Mergify.

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[codecov]

Codecov Report

Merging #2722 (fe668db) into 0.28.x (414bff1) will decrease coverage by 0.01%.
The diff coverage is 34.28%.

@@            Coverage Diff             @@
##           0.28.x    #2722      +/-   ##
==========================================
- Coverage   63.92%   63.92%   -0.01%     
==========================================
  Files         103      103              
  Lines       22306    22310       +4     
  Branches    10796    10796              
==========================================
+ Hits        14259    14261       +2     
  Misses       5827     5827              
- Partials     2220     2222       +2     

Files Changed	Coverage Δ
include/exiv2/image.hpp	100.00% <ø> (ø)
include/exiv2/matroskavideo.hpp	60.00% <ø> (ø)
include/exiv2/slice.hpp	90.56% <ø> (ø)
include/exiv2/value.hpp	85.11% <ø> (ø)
src/basicio.cpp	50.52% <0.00%> (ø)
src/canonmn_int.cpp	72.88% <ø> (ø)
src/nikonmn_int.cpp	61.20% <0.00%> (ø)
src/panasonicmn_int.cpp	52.58% <ø> (ø)
src/quicktimevideo.cpp	60.14% <0.00%> (ø)
src/rafimage.cpp	20.29% <ø> (ø)
... and 13 more