[OpenMP] Add __kmpc_omp_target_task_alloc to dllexport

This patch enables use of the entry on Windows.

Differential Revision: https://reviews.llvm.org/D92618

Author: hansangbae

openmp/runtime/src/dllexports

@@ -387,6 +387,7 @@ kmpc_set_disp_num_buffers                   267
         __kmpc_task_allow_completion_event  276
         __kmpc_taskred_init                 277
         __kmpc_taskred_modifier_init        278
+        __kmpc_omp_target_task_alloc        279
 %endif
 
 # User API entry points that have both lower- and upper- case versions for Fortran.