audio/gsm: Update 1.0.23 => 1.0.24

Changelog:
Fri Mar 6 07:16:24 2026 Jutta Degener (jutta@pobox.com)

• Release 1.0 Patchlevel 24 Left-shifting negative signed integers is undefined behavior as per the C standard; so let's not do that. Thanks to Nanang Izzuddin <nanang@teluu.com> for an exhaustive list of places where we definitely did do that, and for the patience to wait until I managed to Actually Read What the C99 Standard Says, rather than what I _want_ it to say. Also, don't forget the P0() and the void and int return types in a few places of the test code.

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