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

arch/x86/boot/Makefile

@@ -69,6 +69,7 @@ KBUILD_CFLAGS	:= $(REALMODE_CFLAGS) -D_SETUP
 KBUILD_AFLAGS	:= $(KBUILD_CFLAGS) -D__ASSEMBLY__
 KBUILD_CFLAGS	+= $(call cc-option,-fmacro-prefix-map=$(srctree)/=)
 KBUILD_CFLAGS	+= -fno-asynchronous-unwind-tables
+KBUILD_CFLAGS	+= $(CONFIG_CC_IMPLICIT_FALLTHROUGH)
 GCOV_PROFILE := n
 UBSAN_SANITIZE := n
 

arch/x86/boot/printf.c

@@ -246,6 +246,7 @@ int vsprintf(char *buf, const char *fmt, va_list args)
 
 		case 'x':
 			flags |= SMALL;
+			fallthrough;
 		case 'X':
 			base = 16;
 			break;