row_buffer in fwd_txfm2d_c must be 16-byte aligned

row_buffer in fwd_txfm2d_c() is passed as the first argument to
av1_round_shift_array(). av1_round_shift_array_sse4_1() casts its first
parameter to __m128i *. Therefore row_buffer must be 16-byte aligned.

Bug: aomedia:3375
Change-Id: Iceff81a1792edab51f7b823e8f069ae78b2d9597
diff --git a/av1/encoder/av1_fwd_txfm2d.c b/av1/encoder/av1_fwd_txfm2d.c
index d525e29..7693021 100644
--- a/av1/encoder/av1_fwd_txfm2d.c
+++ b/av1/encoder/av1_fwd_txfm2d.c
@@ -105,7 +105,7 @@
     }
   }
 
-  int32_t row_buffer[MAX_TX_SIZE];
+  DECLARE_ALIGNED(16, int32_t, row_buffer[MAX_TX_SIZE]);
 
   // Rows
   for (r = 0; r < txfm_size_row; ++r) {