The `ARG` macro should be uppercased when undefining it

In main, the ARG macro is accidentally lowercased when it is undefined, which results in the macro not actually being undefined:

src/cryptsetup.c:3104

#define ARG(A, B, C, D, E, F, G, H) { A, B, C, NULL, A ## _ID, D, E },
#include "cryptsetup_arg_list.h"
#undef arg

The other two times that this pattern appears (src/cryptsetup.c:47 and src/cryptsetup_args.h:102), it is correct.