ion: Fix build warning

Add #include <linux/device.h> to fix the following warning seen
with gcc 4.7.3:

In file included from drivers/staging/android/ion/ion_heap.c:26:0:
drivers/staging/android/ion/ion_priv.h:358:21: warning: ‘struct device’ declared inside parameter list [enabled by default]
drivers/staging/android/ion/ion_priv.h:358:21: warning: its scope is only this definition or declaration, which is probably not what you want [enabled by default]

Change-Id: Icc249b32d877a5b76b1669c99bef2b05d9e322da
Signed-off-by: John Stultz <john.stultz@linaro.org>