Avoid warnings regarding loss of qualifiers:
warning: cast from type ‘const uint8_t* {aka const unsigned char*}’
to type ‘v16i8* {aka __vector(16) signed char*}’ casts away
qualifiers

BUG=libyuv:793

Change-Id: Ie0d215bc07b49285b5d06ee91ccc2c9a7979799e
Reviewed-on: https://chromium-review.googlesource.com/1107879Reviewed-by: Frank Barchard <fbarchard@chromium.org>
Commit-Queue: Frank Barchard <fbarchard@chromium.org>