cmake: Fix include_directories for exiv2lib target (0.27->master)

[jtojnar]

One more fixup we discovered in Nixpkgs (NixOS/nixpkgs#97161 (comment)).

The headers are installed to ${CMAKE_INSTALL_INCLUDEDIR} but the CMake config was hardcoded to include directory.

[clanmills]

Thanks, Jan. This looks like another great find. Remember my question: "What else lives at the bottom of the swamp?". I'll deal with this tomorrow. Thank You for taking the time to let me know about this.

[piponazo]

It looks good to me. Thanks! 👍

[piponazo]

@clanmills I'll let you merge this PR one you think it is appropriate. These days I have been taking care of merging things in master, but I am not so active in seeing the progress on 0.27-maintenance.

[clanmills]

Don't think about 0.27-maintenance. I'll take care of it!

Thanks very much for your efforts in the last couple of days. I really appreciate you and @neheb working together.

It's good to see the CI working properly on master. I'm about to try to fix it on 0.27-maintenance. I'd really like both 0.27-maintenance and master to build Cygwin64 and MinGW/msys2. If you know how to do that, I'd appreciate your help.