staging: ashmem: Add missing include

Include <linux/types.h> into ashmem.h to ensure referenced types
are defined

Cc: Android Kernel Team <kernel-team@android.com>
Cc: Greg KH <gregkh@linuxfoundation.org>
Signed-off-by: Rom Lemarchand <romlem@android.com>
[jstultz: Minor commit message tweaks]
Signed-off-by: John Stultz <john.stultz@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>