Fix build failure when using Android NDK r20

[ming]

When building Android with NDK r20, libyuv will fail to build with many errors, such as:

In file included from ../../yuv/source/compare.cc:14:
In file included from ndk20/sources/cxx-stl/llvm-libc++/include/math.h:309:
In file included from ndk20/sources/cxx-stl/llvm-libc++/include/type_traits:406:
ndk20/sources/cxx-stl/llvm-libc++/include/cstddef:50:9: error:
      no member named 'ptrdiff_t' in the global namespace
using ::ptrdiff_t;
      ~~^
ndk20/sources/cxx-stl/llvm-libc++/include/cstddef:51:9: error:
      no member named 'size_t' in the global namespace
using ::size_t;
      ~~^

[ming]

In 6032:

Fixed #2207: build failure when using Android NDK r20