Fix braino in arm assembler.

Method is pure virtual. Caught by clang.

Change-Id: I061666ec919702fa7c30e9a98161cad56a9c864d
diff --git a/compiler/utils/arm/assembler_arm.h b/compiler/utils/arm/assembler_arm.h
index 69ecc8f..08d88ab 100644
--- a/compiler/utils/arm/assembler_arm.h
+++ b/compiler/utils/arm/assembler_arm.h
@@ -832,7 +832,7 @@
                                      uint32_t immediate,
                                      ShifterOperand* shifter_op) = 0;
 
-  virtual bool ShifterOperandCanAlwaysHold(uint32_t immediate);
+  virtual bool ShifterOperandCanAlwaysHold(uint32_t immediate) = 0;
 
   static bool IsInstructionForExceptionHandling(uintptr_t pc);