Fix FTBFS with GCC 15

Both compression algorithms and ciphers are stored in function arrays, which aren't declared with all arguments. This is no longer supported in C23, which the default standard for GCC 15. Patch the function declarations so that they use a uniform interface to be specified in such arrays.

This patch doesn't attempt to make all pointers forwarded as arguments agree in signedness, as major changes would be required throughout the code. Such changes aren't strictly required to build with GCC 15 and already affected the existing code for some algorithms and ciphers.

This is split into two patches to make it easier to review and handle in the future.

Closes: #1096497

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Mind that a careful review is important here, as these patches shouldn't change the output of the program in any way. As there is almost no tests available, it becomes harder to ensure that.

PS: I recommend importing the patches as Git commits (e.g. with gbp pq import) and reviewing with git log -p --word-diff=color --word-diff-regex=., so that the changes are easier to understand and verify.