kheaders: Use array declaration instead of char

In the Linux kernel, the following vulnerability has been resolved:

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

...