winpthreads: testcases: Silence a Clang 23 warning about unused-but-set-global

This fixes the following warning (that is treated as an error
in CI):

    ../../tests/pthread_barrier/barrier4.c:50:12: warning: variable 'otherThreadCount' set but not used [-Wunused-but-set-global]
       50 | static int otherThreadCount = 0;
          |            ^

Signed-off-by: Martin Storsjö <martin@martin.st>