rt: Initialize mq_send input on tst-mqueue{5,6}

rt: Initialize mq_send input on tst-mqueue{5,6}

GCC with -Os warns that the mq_send input may be used uninitialized.
Although for the tests the data content sent is not important, since
both tests checks only if mq_notify was properly set, the warning is
correct and data is indeed uninitialized.

Checked on x86_64-linux-gnu and i686-linux-gnu.
Reviewed-by: Carlos O'Donell <carlos@redhat.com>
Tested-by: Carlos O'Donell <carlos@redhat.com>