swiotlb: Don't include linux/swiotlb.h twice in lib/swiotlb.c

There's no point in including the linux/swiotlb.h header twice in
lib/swiotlb.c - this patch gets rid of the unneeded include.

Signed-off-by: Jesper Juhl <jj@chaosbits.net>
Signed-off-by: Ingo Molnar <mingo@elte.hu>