Fix build by putting ATTRIBUTE_UNUSUED.

Change-Id: If894fa1343a7c8a6456d707e815d803404a171d7
diff --git a/compiler/utils/arm/assembler_thumb2.cc b/compiler/utils/arm/assembler_thumb2.cc
index 895cc26..5383c28 100644
--- a/compiler/utils/arm/assembler_thumb2.cc
+++ b/compiler/utils/arm/assembler_thumb2.cc
@@ -25,8 +25,8 @@
 namespace art {
 namespace arm {
 
-bool Thumb2Assembler::ShifterOperandCanHold(Register rd,
-                                            Register rn,
+bool Thumb2Assembler::ShifterOperandCanHold(Register rd ATTRIBUTE_UNUSED,
+                                            Register rn ATTRIBUTE_UNUSED,
                                             Opcode opcode,
                                             uint32_t immediate,
                                             ShifterOperand* shifter_op) {