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

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

sys/net/route/nhop_ctl.c:508:21: error: a function declaration without a prototype is deprecated in all versions of C [-Werror,-Wstrict-prototypes]
alloc_nhop_structure()
                    ^
                     void

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

MFC after: 3 days