Fix warning when building with GCC 5.
Fix warning when building with GCC 5.

These caused the following warning with GCC 5:
     warning: logical not is only applied to the left hand side of
     comparison [-Wlogical-not-parentheses]
     assert(!is_compound == (cm->reference_mode == SINGLE_REFERENCE));

Change-Id: If296aabb2311ceb7d903b395c1549ef81c2cbf9b
(cherry picked from commit c6cf7a6111f6539f01a34865aaedd179a5ae062c)

ShiftMediaProject/libvpx,ShiftMediaProject/libvpx,ShiftMediaProject/libvpx,mwgoldsmith/libvpx,mwgoldsmith/vpx,Topopiccione/libvpx,webmproject/libvpx,mwgoldsmith/vpx,mwgoldsmith/vpx,Topopiccione/libvpx,mwgoldsmith/vpx,mwgoldsmith/libvpx,webmproject/libvpx,Topopiccione/libvpx,mwgoldsmith/libvpx,mwgoldsmith/libvpx,webmproject/libvpx,mwgoldsmith/libvpx,Topopiccione/libvpx,webmproject/libvpx,webmproject/libvpx,Topopiccione/libvpx,mwgoldsmith/vpx,Topopiccione/libvpx,webmproject/libvpx,mwgoldsmith/libvpx,ShiftMediaProject/libvpx,ShiftMediaProject/libvpx,mwgoldsmith/vpx