tools/bootconfig: Fix a build error accroding to undefined fallthrough

Since the "fallthrough" is defined only in the kernel, building
lib/bootconfig.c as a part of user-space tools causes a build
error.

Add a dummy fallthrough to avoid the build error.

Link: https://lkml.kernel.org/r/162087519356.442660.11385099982318160180.stgit@devnote2

Cc: Ingo Molnar <mingo@kernel.org>
Cc: stable@vger.kernel.org
Fixes: 4c1ca831

 ("Revert "lib: Revert use of fallthrough pseudo-keyword in lib/"")
Signed-off-by: Masami Hiramatsu <mhiramat@kernel.org>
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>