uml: mmapper_kern needs MODULE_LICENSE

Add missing MODULE_LICENSE():

WARNING: modpost: missing MODULE_LICENSE() in arch/um/drivers/mmapper_kern.o

Signed-off-by: Randy Dunlap <randy.dunlap@oracle.com>
Cc: Greg Lonnon <glonnon@ridgerun.com>
Cc: Jeff Dike <jdike@addtoit.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>