Fix for duplicate define
../../third_party/libyuv/include/libyuv/scale_row.h:29:9: error: 'LIBYUV_DISABLE_X86' macro redefined [-Werror,-Wmacro-redefined]
        ^

GYP version relys on headers disabling the optimization.
This CL does the same for BUILD.gn
TBR=kjellander@chromium.org
BUG=libyuv:625

Review URL: https://codereview.chromium.org/2149823003 .