highbd_pickrst_neon.c: implicit conversion fix

Fix for C4244 warning of MSVC where int64_t was implicitly converted
to int16_t in function: highbd_find_average_neon.

Bug: aomedia:3507
Change-Id: I65de50bd1e9680d663f711a2eef9f09d9b5b4541

av1/encoder/arm/neon/highbd_pickrst_neon.c

diff --git a/av1/encoder/arm/neon/highbd_pickrst_neon.c b/av1/encoder/arm/neon/highbd_pickrst_neon.c
index 72b1f62..76e0344 100644
--- a/av1/encoder/arm/neon/highbd_pickrst_neon.c
+++ b/av1/encoder/arm/neon/highbd_pickrst_neon.c
@@ -337,7 +337,7 @@
 
     src += src_stride;
   } while (--h != 0);
-  return (horizontal_add_s64x2(sum_s64) + sum) / (height * width);
+  return (int16_t)((horizontal_add_s64x2(sum_s64) + sum) / (height * width));
 }
 
 static INLINE void compute_H_one_col(int16x8_t *dgd, int col, int64_t *H,