external/parallel_string_radix_sort.h: do not include <omp.h>
external/parallel_string_radix_sort.h: do not include <omp.h>

Looks like the inclusion of <omp.h> header is not required here, as the
file only uses openmp pragmas.

Fixed compilation with LLVM in some environment.

rantala/string-sorting,rantala/string-sorting,rantala/string-sorting,rantala/string-sorting