[openmp] [test] Add missing <limits> include to capacity_nthreads

[mgorny]

Summary

Otherwise the build against libstdc++ fails:

$ "clang++" "-fopenmp" "-pthread" "-I" "/home/mgorny/git/llvm-project/openmp/runtime/test" "-I" "/home/mgorny/git/llvm-project/build.libomp/runtime/src" "-L" "/home/mgorny/git/llvm-project/build.libomp/runtime/src" "-I" "/home/mgorny/git/llvm-project/openmp/runtime/test/ompt" "-std=c++14" "/home/mgorny/git/llvm-project/openmp/runtime/test/tasking/hidden_helper_task/capacity_nthreads.cpp" "-o" "/home/mgorny/git/llvm-project/build.libomp/runtime/test/tasking/hidden_helper_task/Output/capacity_nthreads.cpp.tmp" "-lm" "-latomic"
# command stderr:
/home/mgorny/git/llvm-project/openmp/runtime/test/tasking/hidden_helper_task/capacity_nthreads.cpp:12:31: error: no member named 'numeric_limits' in namespace 'std'
                         std::numeric_limits<int>::max());
                         ~~~~~^
/home/mgorny/git/llvm-project/openmp/runtime/test/tasking/hidden_helper_task/capacity_nthreads.cpp:12:49: error: expected '(' for function-style cast or type construction
                         std::numeric_limits<int>::max());
                                             ~~~^
/home/mgorny/git/llvm-project/openmp/runtime/test/tasking/hidden_helper_task/capacity_nthreads.cpp:12:52: error: no member named 'max' in the global namespace
                         std::numeric_limits<int>::max());
                                                 ~~^
3 errors generated.

error: command failed with exit status: 1

[tianshilei1992]

LGTM. Thanks!

openmp/runtime/test/tasking/hidden_helper_task/capacity_nthreads.cpp

 // RUN: %libomp-cxx-compile-and-run
 
 #include <omp.h>
 
 #include <algorithm>
 #include <cassert>
+#include <limits>
 #include <vector>
 
 int main(int argc, char *argv[]) {
   const int N = std::min(std::max(std::max(32, 4 * omp_get_max_threads()),
                                   4 * omp_get_num_procs()),
                          std::numeric_limits<int>::max());
 
   std::vector<int> data(N);