[OpenMP version] Tiny issues in stencil/convolution-3d benchmark and general polybench library

[NoyaFangzhou]

Hi, I found a tiny problem on stencil/convolution-3d benchmark. In the init_array and print_array function, the inner most loop index variable should be k, not j. In addition, in utility/polybench.c, it doesn't includes polybench.h header at the beginning, which may cause some issues when running.