nav2_constrained_smoother: Use unsigned char instead of non-standard u_char type

[traversaro]

Basic Info

Info	Please fill out this column
Ticket(s) this addresses	No ticket.
Primary OS tested on	Windows.
Robotic platform tested on	This is just a compilation fix, no runtime changes.
Does this PR contain AI generated software?	No.

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Description of contribution in a few bullet points

u_char is not a standard C or C++ data type, while unsigned char is equivalent to u_char and is a standard data type. This change permits to compile nav2_constrained_smoother on Windows.

Description of documentation updates required from your changes

None.

Description of how this change was tested

I checked if the compilation was successful.

For Maintainers:

• Check that any new parameters added are updated in docs.nav2.org
• Check that any significant change is added to the migration guide
• Check that any new features OR changes to existing behaviors are reflected in the tuning guide
• Check that any new functions have Doxygen added
• Check that any new features have test coverage
• Check that any new plugins is added to the plugins page
• If BT Node, Additionally: add to BT's XML index of nodes for groot, BT package's readme table, and BT library lists

[traversaro]

It seems that clang-format change the suggestion w.r.t. to some lines due to the additional length of unsigned char vs u_char, I will look into that.

[SteveMacenski]

@traversaro CI, linting, and DCO are unhappy. Once you resolve, happy to merge :-)

[traversaro]

DCO and linting fixed. I still need to understand more about the CI test failure.