vpx_sum_squares_2d_i16_neon(): Make |s2| a uint64x1_t.

vpx_sum_squares_2d_i16_neon(): Make |s2| a uint64x1_t.

This fixes the build with at least GCC 7.3, where it was previously failing
with:

sum_squares_neon.c: In function 'vpx_sum_squares_2d_i16_neon':
sum_squares_neon.c: note: use -flax-vector-conversions to permit conversions between vectors with differing element types or numbers of subparts
     s2 = vpaddl_u32(s1);
     ^~
sum_squares_neon.c: incompatible types when assigning to type 'int64x1_t' from type 'uint64x1_t'
     s2 = vpaddl_u32(s1);
        ^
sum_squares_neon.c: incompatible types when assigning to type 'int64x1_t' from type 'uint64x1_t'
     s2 = vadd_u64(vget_low_u64(s1), vget_high_u64(s1));
        ^
sum_squares_neon.c: incompatible type for argument 1 of 'vget_lane_u64'
   return vget_lane_u64(s2, 0);
                        ^~

The generated assembly was verified to remain identical with both GCC and
LLVM.

Bug: chromium:819249
Change-Id: I2778428ee1fee0a674d0d4910347c2a717de21ac