Adjust function definition in kern_poll.c to avoid clang 15 warning

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

sys/kern/kern_poll.c:374:16: error: a function declaration without a prototype is deprecated in all versions of C [-Werror,-Wstrict-prototypes]
netisr_pollmore()
               ^
                void

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

MFC after: 3 days