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

sys/isa/pnp.c

@@ -115,7 +115,7 @@ pnp_write(int d, u_char r)
  * Intel May 94.
  */
 static void
-pnp_send_initiation_key()
+pnp_send_initiation_key(void)
 {
 	int cur, i;