Fix build breakage in dwarf_test.

Define a variable only if it's going to be used.

Change-Id: Id293f79bf986c79fb33daa3242388303c0fb1f08
diff --git a/compiler/dwarf/dwarf_test.cc b/compiler/dwarf/dwarf_test.cc
index 4971f0e..4d423d0 100644
--- a/compiler/dwarf/dwarf_test.cc
+++ b/compiler/dwarf/dwarf_test.cc
@@ -26,11 +26,11 @@
 namespace art {
 namespace dwarf {
 
-constexpr CFIFormat kCFIFormat = DW_DEBUG_FRAME_FORMAT;
-
 // Run the tests only on host since we need objdump.
 #ifndef HAVE_ANDROID_OS
 
+constexpr CFIFormat kCFIFormat = DW_DEBUG_FRAME_FORMAT;
+
 TEST_F(DwarfTest, DebugFrame) {
   const bool is64bit = false;