include/linux/fb.h: Add a missing include for 'list.h'

The modelist data uses the list definition but the 'list.h' header
were not being included. The build failure is bellow:

,----
| In file included from yyyy.c:16:0:
| .../u-boot/include/linux/fb.h:503:19: error: field 'modelist' has incomplete type
|   struct list_head modelist; /* mode list */
|                    ^
| make[1]: *** [yyyy.o] Error 1
| make[1]: Leaving directory `.../u-boot/board/xxx/yyyy'
| make: *** [board/xxx/yyyy/libyyyy.o] Error 2
`----

Signed-off-by: Otavio Salvador <otavio@ossystems.com.br>