the built in __msa_ld_b() expects a void * without const.
Cast pointers to void * to avoid build warning.

TBR=johannkoenig@google.com
Bug: libyuv:805
Change-Id: Iabc4820ecf4a3a7dcb0063e67ce276ae2a4f0501
Tested: gn gen out/Release "--args=is_debug=false target_os=\"android\" target_cpu=\"mips64el\" mips_arch_variant=\"r6\" mips_use_msa=true is_component_build=true is_clang=true"
ninja -v -C out/Release libyuv_unittest
Reviewed-on: https://chromium-review.googlesource.com/1125400
Commit-Queue: Frank Barchard <fbarchard@chromium.org>
Reviewed-by: Frank Barchard <fbarchard@chromium.org>