staging: sw_sync: Add stubs for kernels without CONFIG_SW_SYNC

Add stubs for kernels without CONFIG_SW_SYNC

Cc: Colin Cross <ccross@android.com>
Cc: Android Kernel Team <kernel-team@android.com>
Signed-off-by: Greg Hackmann <ghackmann@google.com>
[jstultz: resolved minor conflict, tweaked commit message]
Signed-off-by: John Stultz <john.stultz@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>