x86/boot: Add a fallthrough annotation

Add implicit fallthrough checking to the decompressor code and fix this
warning:

  arch/x86/boot/printf.c: In function ‘vsprintf’:
  arch/x86/boot/printf.c:248:10: warning: this statement may fall through [-Wimplicit-fallthrough=]
    248 |    flags |= SMALL;
        |          ^
  arch/x86/boot/printf.c:249:3: note: here
    249 |   case 'X':
        |   ^~~~

This is a patch from three years ago which I found in my trees, thus the
SUSE authorship still.

Signed-off-by: Borislav Petkov <bp@suse.de>
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Link: https://lore.kernel.org/r/20240516102240.16270-1-bp@kernel.org