driver core: export subsys_virtual_register

Modules want to call this function, so it needs to be exported.

Reported-by: Daniel Mack <zonque@gmail.com>
Cc: Kay Sievers <kay@vrfy.org>
Cc: Tejun Heo <tj@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>