kheaders: Use array declaration instead of char

kheaders: Use array declaration instead of char

Under CONFIG_FORTIFY_SOURCE, memcpy() will check the size of destination
and source buffers. Defining kernel_headers_data as "char" would trip
this check. Since these addresses are treated as byte arrays, define
them as arrays (as done everywhere else).

This was seen with:

  $ cat /sys/kernel/kheaders.tar.xz >> /dev/null

  detected buffer overflow in memcpy
  kernel BUG at lib/string_helpers.c:1027!
  ...
  RIP: 0010:fortify_panic+0xf/0x20
  [...]
  Call Trace:
   <TASK>
   ikheaders_read+0x45/0x50 [kheaders]
   kernfs_fop_read_iter+0x1a4/0x2f0
  ...

Reported-by: Jakub Kicinski <kuba@kernel.org>
Link: https://lore.kernel.org/bpf/20230302112130.6e402a98@kernel.org/
Acked-by: Joel Fernandes (Google) <joel@joelfernandes.org>
Reviewed-by: Alexander Lobakin <aleksander.lobakin@intel.com>
Tested-by: Jakub Kicinski <kuba@kernel.org>
Fixes: 43d8ce9d65a5 ("Provide in-kernel headers to make extending kernel easier")
Cc: stable@vger.kernel.org
Signed-off-by: Kees Cook <keescook@chromium.org>
Link: https://lore.kernel.org/r/20230302224946.never.243-kees@kernel.org