ANDROID: dma-buf: cma_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.

Cc: John Stultz <jstultz@google.com>
Cc: T.J. Mercier <tjmercier@google.com>
Cc: Eric Biggers <ebiggers@google.com>
Fixes: a6ccb03f ("dma-buf: dma-heap: export declared functions")
Fixes: a8d66d53 ("ANDROID: dma-buf: heaps: Allow cma heaps to be configured as a module")
Change-Id: I39fc36296f96772bff0f9f214dc6379d02ab8958
Signed-off-by: Carlos Llamas <cmllamas@google.com>