BACKPORT: staging: goldfish: audio: fix compiliation on arm

We do actually need slab.h, by luck we get it on other platforms but not
always on ARM. Include it properly.

Signed-off-by: Greg Hackmann <ghackmann@google.com>
Signed-off-by: Jin Qian <jinqian@android.com>
Signed-off-by: Alan <alan@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
(cherry picked from commit 4532150762ceb0d6fd765ebcb3ba6966fbb8faab)

Change-Id: I93a0c35da40f26aaa7c253e3c0cefaa883ea3391