Change the syntax of saving result of ::omp_get_num_threads()

[sbastrakov]

The previously used syntax was valid but sometimes caused internal error for Intel compiler in CI:

/home/runner/work/alpaka/alpaka/include/alpaka/kernel/TaskKernelCpuOmp2Threads.hpp(134): internal error: assertion failed: cd_new_hash_table: malloc failed (cdhashtbl.c, line 134 in cd_new_hash_table)

                                  int const numThreads(::omp_get_num_threads());

The changed one suggested by @bernhardmgruber appears to work around the issue.

I faced this issue sometimes with #1336 and after making the fix in that branch it no longer happened, so I believe it indeed works around the issue.

[sbastrakov]

Ready to be merged.