sad_test: fix vp8-only build

ROUND_POWER_OF_TWO() is defined in vp9 headers currently, avoid it in
non-high-bitdepth code

Change-Id: Ic28b8f95ef7964800475ee8b35be5f9cea9afab6
diff --git a/test/sad_test.cc b/test/sad_test.cc
index 615aad4..114f0af 100644
--- a/test/sad_test.cc
+++ b/test/sad_test.cc
@@ -234,7 +234,7 @@
 #else
         const int tmp = second_pred[h * width_ + w] +
             reference[h * reference_stride_ + w];
-        const uint8_t comp_pred = ROUND_POWER_OF_TWO(tmp, 1);
+        const uint8_t comp_pred = (tmp + 1) >> 1;
         sad += abs(source[h * source_stride_ + w] - comp_pred);
 #endif
       }