zlib: annotate switch/case fallthrough cases

zlib: annotate switch/case fallthrough cases

The inflate state machine in zlib uses switch/case fall-through's
extensively, as it sometimes advances the state, and lets the
conveniently placed next case statement handle the new state already.
The pattern here is:
		state->mode = LEN;
	case LEN:

Annotate those occasions with the "fallthrough;" macro, to let compilers
know this is fine when using -Wimplicit-fallthrough.

This mimics the upstream commit 76f70abbc73f:
Author:  Mark Adler <madler@alumni.caltech.edu>
Date:    Sun Mar 27 00:12:38 2022 -0700
Subject: Add fallthrough comments for gcc.

Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Link: https://github.com/madler/zlib/commit/76f70abbc73f
Reviewed-by: Tom Rini <trini@konsulko.com>