ANDROID: dma-buf: system_heap: import DMA_BUF_HEAP namespace

The dma-buf heap symbols used in this module have been exported upstream
through the "DMA_BUF_HEAP" namespace in commit a6ccb03f ("dma-buf:
dma-heap: export declared functions"). Import the namespace accordingly.

Fixes: a6ccb03f ("dma-buf: dma-heap: export declared functions")
Change-Id: Ibab80a2021e1f125923d13817da68a764e92646a
Signed-off-by: Carlos Llamas <cmllamas@google.com>