iolog: fix error when compiled as c++

iolog: fix error when compiled as c++

Given the main.cc source file:
  #include <fio.h>
  int main() { return 0; }

And the gcc command line:
  gcc main.cc -Ifio

In file included from fio/stat.h:4:0,
                 from fio/thread_options.h:7,
                 from fio/fio.h:18,
                 from main.cc:1:
fio/iolog.h: In function 'io_sample* __get_sample(void*, int,
uint64_t)':
fio/iolog.h:125:53: warning: pointer of type 'void *' used in arithmetic
[-Wpointer-arith]
  return samples + sample * __log_entry_sz(log_offset);
                                                     ^
fio/iolog.h:125:17: error: invalid conversion from 'void*' to
'io_sample*' [-fpermissive]
  return samples + sample * __log_entry_sz(log_offset);

Signed-off-by: Casey Bodley <cbodley@redhat.com>