staging: sw_sync: add stubs for kernels without CONFIG_SW_SYNC

Change-Id: I7a12679eb3c10b1beaf36c6612cf2e3f6662a034
Signed-off-by: Greg Hackmann <ghackmann@google.com>
Git-commit: 3eeae661f1c26431dd03d2760e2373620bb488ca
Git-Repo: https://android.googlesource.com/kernel/common.git


[imaund@codeaurora.org: Resolve merge conflicts]
Signed-off-by: Ian Maund <imaund@codeaurora.org>