Bug#21680094: INCORRECT DECLARATION IN CLIENT_PLUGIN.H

Change the declaration of the deinit function pointer in the
client plugin header from "int (*deinit)();" to
"int (*deinit)(void);" so that it is a pointer to a function
with no arguments rather than a pointer to a function with
an arbitrary number of arguments.

(cherry picked from commit 2d91dd4bd9e11022e81c17415e289d4bb8b0b88f)