workqueue: rename private to priv for compiling as c++

workqueue: rename private to priv for compiling as c++

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

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

In file included from ./ioengine.h:10:0,
                 from ./iolog.h:7,
                 from ./stat.h:4,
                 from ./thread_options.h:7,
                 from ./fio.h:18,
                 from main.cc:1:
./workqueue.h:19:8: error: expected unqualified-id before ‘private’
  void *private;
        ^
./workqueue.h:19:7: error: expected ‘;’ at end of member declaration
  void *private;
       ^
./workqueue.h:19:15: error: expected ‘:’ before ‘;’ token
  void *private;
               ^

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