[buddy] workaround newer clang warning

It seems clang now warn about fallthrough case statements in C, but
ignore any /* fall through */ comment if that comes from a macro.

* src/bddop.c: Use the fallthrough attribute if available.
9 jobs for adl/ltlsynt-csv in 50 minutes and 53 seconds (queued for 1 second)
Status Job ID Name Coverage
  Build
canceled #91073
alpine-gcc

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passed #91074
arch-clang

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canceled #91075
arch-gcc-glibcxxdebug

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allowed to fail
debian-gcc-snapshot

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canceled #91070
debian-stable-gcc

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debian-unstable-gcc-coverage

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90.5%
passed #91076
mingw-shared

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mingw-static

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armv7
raspbian

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