lib_nbgl: Constify nbgl_layoutTagValueList_t pairs element

[xchapron-ledger]

Description

lib_nbgl: Constify nbgl_layoutTagValueList_t pairs element
this allow to use const pairs, and therefore pairs from flash without having to drop the const qualifier.

Changes include

• Bugfix (non-breaking change that solves an issue)
• New feature (non-breaking change that adds functionality)
• Breaking change (change that is not backwards-compatible and/or changes current functionality)
• Tests
• Documentation
• Other (for changes that might not fit in any category)

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (4569539) 69.09% compared to head (4a02f17) 69.09%.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #460   +/-   ##
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  Coverage   69.09%   69.09%           
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  Files          11       11           
  Lines         877      877           
=======================================
  Hits          606      606           
  Misses        271      271           

Flag	Coverage Δ
unittests	69.09% <ø> (ø)

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