Really fix the mac build.

I forgot an underscore.

Test: Builds.
Change-Id: Ib7da13dea315822af89c53a569fb0a0c2fd5d321
diff --git a/benchmarks/util.cpp b/benchmarks/util.cpp
index 91cdf7d..d9641cf 100644
--- a/benchmarks/util.cpp
+++ b/benchmarks/util.cpp
@@ -55,7 +55,7 @@
   return buf_aligned;
 }
 
-#if defined(__APPLE_)
+#if defined(__APPLE__)
 
 // Darwin doesn't support this, so do nothing.
 bool LockToCPU(int) {