include/linux/dmapool.h: declare struct device

dmapool uses struct device in function arguments but relies on an
implicit inclusion to declare struct device causing warnings in some
configurations:

  include/linux/dmapool.h:31:7: warning: 'struct device' declared inside parameter list

Fix this by adding a struct device declaration to the file.

Signed-off-by: Mark Brown <broonie@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>