deskutils/plan: Fix LLVM16 build

Fix the following LLVM16 error:

main.c:126:9: error: incompatible function pointer types passing 'void (int)' to parameter of type 'void (* _Nonnull)(void)' [-Wincompatible-function-pointer-types]

atexit(plan_exit);
       ^~~~~~~~~

/usr/include/stdlib.h:92:29: note: passing argument to parameter here
int atexit(void (* _Nonnull)(void));

^

MFH: 2023Q2