Adjust function definition in isa's pnp.c to avoid clang 15 warning

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

sys/isa/pnp.c:118:24: error: a function declaration without a prototype is deprecated in all versions of C [-Werror,-Wstrict-prototypes]
pnp_send_initiation_key()
                       ^
                        void

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

MFC after: 3 days