fix CONFIG_SPATIAL_SVC warning

this change checks that CONFIG_SPATIAL_SVC is defined and adds a TODO to
ensure this is changed in the future as the release headers can't
depend on vpx_config.h.

vpx/vpx_encoder.h:164:5: warning: "CONFIG_SPATIAL_SVC" is not defined
[-Wundef]

Change-Id: I797a0150e5f56caf048e7ee00b282fbc9c5ede19
Merge "fix CONFIG_SPATIAL_SVC warning"