WriteCoreDumpFunction is a function pointer that points to the function used to
write content in "buf" into core file, so "buf" should be const-qualify.

Signed-off-by: Qiao Nuohan <qiaonuohan@cn.fujitsu.com>
Reviewed-by: Laszlo Ersek <lersek@redhat.com>
Signed-off-by: Luiz Capitulino <lcapitulino@redhat.com>