[OpenMP] Add a missing dllexport for the new function __kmpc_fork_call_if

This new function was added in b72f1ec,
but wasn't exported from the DLL on Windows.

This fixes the parallel/omp_parallel_if.c OpenMP testcase on
Windows.

Author: mstorsjo

openmp/runtime/src/dllexports

@@ -228,6 +228,7 @@
     __kmpc_for_static_init_4                136
     __kmpc_for_static_init_8                137
     __kmpc_fork_call                        138
+    __kmpc_fork_call_if                     294
     __kmpc_global_num_threads               140
     __kmpc_global_thread_num                141
     __kmpc_in_parallel                      142