[RFC]: Add C implementation for `@stdlib/math/base/special/ellipk`

[Pranavchiku]

Description

This RFC proposes adding C implementation for @stdlib/math/base/special/ellipk.

double stdlib_base_ellipk( const double m );

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• I have read and understood the Code of Conduct.
• Searched for existing issues and pull requests.
• The issue name begins with RFC:.