7603 xuio_stat_wbuf_* should be declared (void)

illumos/illumos-gate@99aa8b55058e512798eafbd71f72f916bdc10181
https://github.com/illumos/illumos-gate/commit/99aa8b55058e512798eafbd71f72f916bdc10181

https://www.illumos.org/issues/7603

The funcs are declared k&r style, where the args are not specified:

void xuio_stat_wbuf_copied();
They should be declared to take no arguments:

void xuio_stat_wbuf_copied(void);
Need to change both .c and .h.

Author: Prashanth Sreenivasa <pks@delphix.com>
Reviewed by: Matthew Ahrens <mahrens@delphix.com>
Reviewed by: Paul Dagnelie <pcd@delphix.com>
Reviewed by: Robert Mustacchi <rm@joyent.com>
Approved by: Richard Lowe <richlowe@richlowe.net>