Adjust function definition in nd6.c to avoid clang 15 warnings

With clang 15, the following -Werror warning is produced:

sys/netinet6/nd6.c:247:12: error: a function declaration without a prototype is deprecated in all versions of C [-Werror,-Wstrict-prototypes]
nd6_destroy()
           ^
            void

This is nd6_destroy() is declared with a (void) argument list, but
defined with an empty argument list. Make the definition match the
declaration.

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