Fix array bound warning in transpose ssse3

ss[2] and ss[3] are not used in this function, but it should be
defined as ss[4] to be consistent with other functions calling
transpose_16bit_4x4

Bug: aomedia:2882
Change-Id: I62a3f230ea4add5c5f9eb3cf48bfeca88281bf33
diff --git a/aom_dsp/x86/aom_subpixel_8t_intrin_ssse3.c b/aom_dsp/x86/aom_subpixel_8t_intrin_ssse3.c
index 18e8a2a..b6ca63d 100644
--- a/aom_dsp/x86/aom_subpixel_8t_intrin_ssse3.c
+++ b/aom_dsp/x86/aom_subpixel_8t_intrin_ssse3.c
@@ -826,7 +826,7 @@
                                   const ptrdiff_t src_stride,
                                   uint8_t *const dst,
                                   const int16_t *const filter) {
-  __m128i s[4], ss[2];
+  __m128i s[4], ss[4];
   __m128i temp;
 
   load_8bit_8x4(src, src_stride, s);