[Framework] BaseMatrix:: Remove warnings about overloaded-virtual about add()

[fredroy]

with (since?) gcc15, It is spamming ~7k warnings like:

Sofa/framework/LinearAlgebra/src/sofa/linearalgebra/BaseMatrix.h:79:18: warning: ‘virtual void sofa::linearalgebra::BaseMatrix::add(Index, Index, const sofa::type::Mat2x2f&)’ was hidden [-Woverloaded-virtual=]

[with-all-tests]

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Reviewers will merge this pull-request only if

• it builds with SUCCESS for all platforms on the CI.
• it does not generate new warnings.
• it does not generate new unit test failures.
• it does not generate new scene test failures.
• it does not break API compatibility.
• it is more than 1 week old (or has fast-merge label).

[fredroy]

[ci-build][with-all-tests]

[alxbilger]

I don't know why but the scene tests did not trigger...