IMPORT: slz: define and use a __fallthrough statement for switch/case

IMPORT: slz: define and use a __fallthrough statement for switch/case

When the code is preprocessed first and compiled later, such as when
built under distcc, the "fall through" comments are dropped and warnings
are emitted. Let's use the alternative "fallthrough" attribute instead,
that is supported by versions of gcc and clang that also produce this
warning.

This is libslz upstream commit 0fdf8ae218f3ecb0b7f22afd1a6b35a4f94053e2