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 <[email protected]>
Cc: [email protected]
Fixes: 4c1ca83 ("Revert "lib: Revert use of fallthrough pseudo-keyword in lib/"")
Signed-off-by: Masami Hiramatsu <[email protected]>
Signed-off-by: Steven Rostedt (VMware) <[email protected]>

Author: mhiramat

tools/bootconfig/include/linux/bootconfig.h

@@ -4,4 +4,8 @@
 
 #include "../../../../include/linux/bootconfig.h"
 
+#ifndef fallthrough
+# define fallthrough
+#endif
+
 #endif