Fix compilation error with armv7, x86 (#2660)
Fix compilation error with armv7, x86 (#2660)

Fixes a compilation error, that appears on alpine linux.
 /builds/lmarz/aports/testing/openimageio/src/oiio-Release-2.1.18.0/src/libOpenImageIO/exif.cpp: In function 'void OpenImageIO_v2_1::read_exif_tag(OpenImageIO_v2_1::ImageSpec&, const TIFFDirEntry*, OpenImageIO_v2_1::cspan<unsigned char>, bool, int, std::set<unsigned int>&, const OpenImageIO_v2_1::pvt::TagMap&)':
 /builds/lmarz/aports/testing/openimageio/src/oiio-Release-2.1.18.0/src/libOpenImageIO/exif.cpp:789:20: error: comparison of integer expressions of different signedness: 'unsigned int' and 'OpenImageIO_v2_1::span<const unsigned char, -1>::index_type' {aka 'int'} [-Werror=sign-compare]
   789 |         if (offset >= buf.size()) {
       |
lgritz/oiio,OpenImageIO/oiio,OpenImageIO/oiio,OpenImageIO/oiio,lgritz/oiio,OpenImageIO/oiio,lgritz/oiio,lgritz/oiio