Fix UB caused by uninitialized variable i

[7FM]

Example output of clang:

RTOSBench/tests/mq/mq_workload.c:61:9: warning: relational comparison result unused [-Wunused-comparison]
   61 |         for (i < 0 ; i < NB_WORKLOAD_TASK; i++)
      |              ~~^~~
RTOSBench/tests/mq/mq_workload.c:61:7: warning: variable 'i' is uninitialized when used here [-Wuninitialized]
   61 |         for (i < 0 ; i < NB_WORKLOAD_TASK; i++)
      |              ^
RTOSBench/tests/mq/mq_workload.c:46:11: note: initialize the variable 'i' to silence this warning
   46 |         int32_t i;
      |                  ^
      |                   = 0
RTOSBench/tests/mq/mq_workload.c:123:3: warning: variable 'i' is uninitialized when used here [-Wuninitialized]
  123 |                 i++;
      |                 ^
RTOSBench/tests/mq/mq_workload.c:116:11: note: initialize the variable 'i' to silence this warning
  116 |         int32_t i;
      |                  ^
      |                   = 0
4 warnings generated.

As a consequence, Clang removes the entire loop, and the tests have no workload tasks.