dimlib: make DIMLIB a hidden symbol

According to Tal Gilboa the only benefit from DIM comes from a driver
that uses it. So it doesn't make sense to make this symbol user visible,
instead all drivers that use it should select it (as is already the case
AFAICT).

Signed-off-by: Uwe Kleine-König <uwe@kleine-koenig.org>
Acked-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: David S. Miller <davem@davemloft.net>