BUILD: Fix linking of test_wbc_calls
    
    Client code does not anymore depend on libpthread in master.
    This is a reason why we didn't notice any linking failure
    in master. But the test should be linked with CLIENT_LIBS.
    
      CCLD     test_wbc_calls
    /usr/bin/ld: src/sss_client/test_wbc_calls-common.o: undefined reference
        to symbol 'pthread_mutexattr_setrobust@@GLIBC_2.12'
    //lib/x86_64-linux-gnu/libpthread.so.0: error adding symbols: DSO missing
        from command line
    collect2: error: ld returned 1 exit status
    Makefile:12460: recipe for target 'test_wbc_calls' failed
    
    Reviewed-by: Fabiano Fidêncio <fidencio@redhat.com>
    (cherry picked from commit c369b062182c746849196e495db467198039edf4)