Fix warning in vl.c

Fix warning in vl.c

vl.c calls dma_helper_init, so it needs to include dma.h to get a
definition for it, otherwise we get compiler warnings like:

/home/hch/work/qemu/vl.c: In function 'main':
/home/hch/work/qemu/vl.c:5518: warning: implicit declaration of function 'dma_helper_init'

Signed-off-by: Christoph Hellwig <hch@lst.de>
git-svn-id: svn://svn.savannah.nongnu.org/qemu/trunk@6887 c046a42c-6fe2-441c-8c8c-71466251a162