avfilter/vf_ssim.c: fix build warning for [-Wmain]

avfilter/vf_ssim.c: fix build warning for [-Wmain]

The build warning message:
src/libavfilter/vf_ssim.c: In function ‘ssim_plane_16bit’:
src/libavfilter/vf_ssim.c:246:24: warning: ‘main’ is usually a function [-Wmain]
         const uint8_t *main = td->main_data[c];
                        ^~~~
src/libavfilter/vf_ssim.c: In function ‘ssim_plane’:
src/libavfilter/vf_ssim.c:289:24: warning: ‘main’ is usually a function [-Wmain]
         const uint8_t *main = td->main_data[c];
                        ^~~~

Signed-off-by: Guo, Yejun <yejun.guo@intel.com>