Fix a compiler warning of unused variable

Change-Id: I4a2faa32cc0847fe14dd8f40156163f4713055ca
diff --git a/aom_dsp/blend_a64_hmask.c b/aom_dsp/blend_a64_hmask.c
index 5ebbe0b..dc11e80 100644
--- a/aom_dsp/blend_a64_hmask.c
+++ b/aom_dsp/blend_a64_hmask.c
@@ -49,6 +49,7 @@
   uint16_t *dst = CONVERT_TO_SHORTPTR(dst_8);
   const uint16_t *src0 = CONVERT_TO_SHORTPTR(src0_8);
   const uint16_t *src1 = CONVERT_TO_SHORTPTR(src1_8);
+  (void)bd;
 
   assert(IMPLIES(src0 == dst, src0_stride == dst_stride));
   assert(IMPLIES(src1 == dst, src1_stride == dst_stride));