[dist-8x8] Fix missing if condition in previous commit
[dist-8x8] Fix missing if condition in previous commit

"if (x->using_dist_8x8)" was missing in the commit to master:
bdedd9878ac6e4309df8c17ad6de4d0e217280e3

Change-Id: I57c0033425afd76dfd90ea9a7116c878a56f7bc8

GrokImageCompression/aom,GrokImageCompression/aom,GrokImageCompression/aom,GrokImageCompression/aom,GrokImageCompression/aom,GrokImageCompression/aom