Staging: hv: hv_mouse: remove unneeded function forward declaration

When the code moved around earlier, this function declaration should
have been removed but it wasn't.  Resolve that.

Cc: Hank Janssen <hjanssen@microsoft.com>
Cc: K. Y. Srinivasan <kys@microsoft.com>
Cc: Haiyang Zhang <haiyangz@microsoft.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>