audio/format: fix Type 'struct entry' has incompatible definitions in different translation units

[Coeur]

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When using Xcode, struct entry already exists (from <search.h>), so the one we declare in audio/format.c needs to be named differently.

[Akemi]

we have some contribution rules here https://github.com/mpv-player/mpv/blob/master/DOCS/contribute.md, like commit message and body. the problem and explanation should be part of the commit body.

[Coeur]

@Akemi can we get a review? Thanks

[sfan5]

Sorry for nitpicking but internal types are usually named mp_thing not mpv_thing. Looks good other than that.

[Coeur]

@sfan5 OK, renamed. Good to merge. :)