lguest: add export.h to lguest files for THIS_MODULE/EXPORT_SYMBOL

We need this in advance of the module.h cleanup, or we'll
get compile errors like this:

  CC      drivers/lguest/lguest_device.o
drivers/lguest/lguest_device.c: In function ‘lguest_devices_init’:
drivers/lguest/lguest_device.c:490: error: ‘THIS_MODULE’ undeclared (first use in this function)

Signed-off-by: Paul Gortmaker <paul.gortmaker@windriver.com>