Fix build failure on Clang 16

alsa-project/alsa-tools pull request #17 was opened from mazunki:

Explicitly casting to `GCallback`, defined as `void (*)(void)`, through the `G_CALLBACK` function.

Fixes issue #12 and by extension https://bugs.gentoo.org/show_bug.cgi?id=880997.

cc @capezotte @thesamesam

Request URL : https://github.com/alsa-project/alsa-tools/pull/17 Patch URL : https://github.com/alsa-project/alsa-tools/pull/17.patch Repository URL: https://github.com/alsa-project/alsa-tools