openmp: fix FLOP/s calculation

[milthorpe]

The GFLOP/s calculation in printTimings() for the OpenMP implementation was reporting incorrect (negative) values for large inputs, due to overflow. Computing ops_per_wg using long (rather than 32-bit) ints fixes the problem.

To test, run e.g.

./bude --deck ../data/bm2

before this change, miniBUDE reported:

...
Running C/OpenMP
- Total time:     30353.730 ms
- Average time:   3794.216 ms
- Interactions/s: 123.319 billion
- GFLOP/s:        -61.996
- GFInst/s:       3082.980

after the change, it reports:

Running C/OpenMP
- Total time:     30727.736 ms
- Average time:   3840.967 ms
- Interactions/s: 121.818 billion
- GFLOP/s:        3660.127
- GFInst/s:       3045.456