fix the generation of the vcd trace

[bouazizrahma]

We use a mutex to protect the index of the array containing the vcd trace.

[codecov-io]

Codecov Report

Merging #35 into master will not change coverage.
The diff coverage is 0%.

@@          Coverage Diff          @@
##           master    #35   +/-   ##
=====================================
  Coverage       0%     0%           
=====================================
  Files          19     19           
  Lines        2937   2872   -65     
=====================================
+ Misses       2937   2872   -65

Impacted Files	Coverage Δ
src/po_hi_protected.c	0% <ø> (ø)	⬆️
src/po_hi_utils.c	0% <ø> (ø)	⬆️
src/po_hi_gqueue.c	0% <0%> (ø)	⬆️
src/po_hi_task.c	0% <0%> (ø)	⬆️
src/po_hi_main.c	0% <0%> (ø)	⬆️

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