ART: Mac build fix

Unused parameter on Mac.

Bug: 18596910
Change-Id: If0fab0b58173997d584fcdce195361d792f1098d
diff --git a/compiler/utils/swap_space.cc b/compiler/utils/swap_space.cc
index fc51435..325ee4f 100644
--- a/compiler/utils/swap_space.cc
+++ b/compiler/utils/swap_space.cc
@@ -153,9 +153,9 @@
   maps_.push_back(new_chunk);
   return new_chunk;
 #else
-  UNUSED(kMininumMapSize);
+  UNUSED(min_size, kMininumMapSize);
   LOG(FATAL) << "No swap file support on the Mac.";
-  return {nullptr, 0U};  // NOLINT [readability/braces] [4]
+  UNREACHABLE();
 #endif
 }