a11y-keyboard-atspi: Fix memory leak

Fix fairly large memory leak when beeping on keys while caps lock is
enabled.  The libatspi2 docs and API were quite misleading, so I
overlooked the fact the event parameter should be freed in the
callback.

This changes the constness of the callback argument, which is new in
libatspi2 2.40 -- yet the actual behavior didn't change, only the
qualifier was removed, see [1].
This might however bring up a compiler warning when building against
libatspi2 < 2.40;  but on the other hand it fixed build with
clang >= 16, see #399.  As it is unlikely to build with clang >= 16
and libatspi2 < 2.40, I think it's a good compromise.

[1] https://gitlab.gnome.org/GNOME/at-spi2-core/-/commit/7dfb0b7fc2d1710ef7fad54f910fa4c6a5e3af17

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