Fix compile-time issue involving implicit declaration of mapclut_paletee()

[rhymeswithmogul]

This commit fixes a compile-time error regarding an implicit declaration of mapclut_paletee() on some compilers and compiler versions. Notably, Arch Linux and Ubuntu 24.10 seem to be affected.

The error resolved is:

../src/lib_ccx/ocr.c: In function 'ocr_rect':
../src/lib_ccx/ocr.c:922:9: error: implicit declaration of function 'mapclut_paletee' [-Wimplicit-function-declaration]
  922 |         mapclut_paletee(palette, alpha, (uint32_t *)rect->data1, rect->nb_colors);
      |         ^~~~~~~~~~~~~~~

This was resolved by #include-ing "ccx_encoders_spupng.h" in the file src/lib_ccx/ocr.c. Thanks to GitHub user @steel-bucket for sharing the fix in this issue's comments.

I was having the same issue, and making this change to that file allowed it to compile on my system (Ubuntu 24.10, GCC 14.2.0-4ubuntu2).

Fixes: #1646

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