coredump: Don't compile flat_core_dump when coredumps are disabled

coredump: Don't compile flat_core_dump when coredumps are disabled

Recently the kernel test robot reported:
> In file included from include/linux/kernel.h:29,
>                     from fs/binfmt_flat.c:21:
>    fs/binfmt_flat.c: In function 'flat_core_dump':
> >> fs/binfmt_flat.c:121:50: error: invalid use of undefined type 'struct coredump_params'
>      121 |                 current->comm, current->pid, cprm->siginfo->si_signo);
>          |                                                  ^~
>    include/linux/printk.h:418:33: note: in definition of macro 'printk_index_wrap'
>      418 |                 _p_func(_fmt, ##__VA_ARGS__);                           \
>          |                                 ^~~~~~~~~~~
>    include/linux/printk.h:499:9: note: in expansion of macro 'printk'
>      499 |         printk(KERN_WARNING pr_fmt(fmt), ##__VA_ARGS__)
>          |         ^~~~~~
>    fs/binfmt_flat.c:120:9: note: in expansion of macro 'pr_warn'
>      120 |         pr_warn("Process %s:%d received signr %d and should have core dumped\n",
>          |         ^~~~~~~
>    At top level:
>    fs/binfmt_flat.c:118:12: warning: 'flat_core_dump' defined but not used [-Wunused-function]
>      118 | static int flat_core_dump(struct coredump_params *cprm)
>          |            ^~~~~~~~~~~~~~

The little dinky do nothing function flat_core_dump has always been
compiled unconditionally.  With my change to move coredump_params into
coredump.h coredump_params reasonably becomes unavailable when
coredump support is not compiled in.  Fix this old issue by simply not
compiling flat_core_dump when coredump support is not supported.

Fixes: a99a3e2efaf1 ("coredump: Move definition of struct coredump_params into coredump.h")
Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>