libvpx

Fix MSVC warning.

A new set of MSVC warnings were introduced by change
I3f36d3f7cd8d15195a6e2fafd1777cdaf9ecb847

In particular MSVC does not like:-

typedef const int16_t subpel_kernel[SUBPEL_TAPS];

struct subpix_fn_table {
  const subpel_kernel *filter_x;
  const subpel_kernel *filter_y;
};

causes  new warning in MSVC.
warning C4114: same type qualifier used more than once

Change-Id: Iae596fd13aadf36169faf00c68eabe9a32a9b156