xen: fix implicit stat.h/module.h include usage in xen-selfballoon.c

xen: fix implicit stat.h/module.h include usage in xen-selfballoon.c

The module.h cleanup has revealed that this file had the following
implicit dependencies as follows:

  CC      drivers/xen/xen-selfballoon.o
drivers/xen/xen-selfballoon.c:272: error: ‘S_IRUGO’ undeclared here (not in a function)
drivers/xen/xen-selfballoon.c:272: error: ‘S_IWUSR’ undeclared here (not in a function)
drivers/xen/xen-selfballoon.c:485: warning: type defaults to ‘int’ in declaration of ‘MODULE_LICENSE’
drivers/xen/xen-selfballoon.c:485: warning: function declaration isn’t a prototype
make[3]: *** [drivers/xen/xen-selfballoon.o] Error 1

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