external/webp: enable WEBP_USE_THREAD

this serializes dsp function pointer initialization avoiding the
potential for multiple initialization or code reordering in the
non-threaded case. if the latter were to occur there would be a race
condition where the pointers had the potential to be uninitialized,
resulting in a crash.

Test: aosp_arm-eng aosp_arm64-eng aosp_x86-eng aosp_x86_64-eng
      aosp_crosshatch-userdebug build

Bug: https://crbug.com/webp/419
Bug: b/139274551
Change-Id: Iaecdf4718820d304d216fc7583ef2b0130a64e9a
1 file changed