Fix the build with respect to new VIXL.

Test: Build ART for ARM64.
Change-Id: I2a9ebf145c61db9f8ceec6616963bac3ad5a7eb9
diff --git a/compiler/optimizing/code_generator_arm64.h b/compiler/optimizing/code_generator_arm64.h
index 5a87615..7201e59 100644
--- a/compiler/optimizing/code_generator_arm64.h
+++ b/compiler/optimizing/code_generator_arm64.h
@@ -459,7 +459,7 @@
 
   // We want to use the STP and LDP instructions to spill and restore registers for slow paths.
   // These instructions can only encode offsets that are multiples of the register size accessed.
-  uint32_t GetPreferredSlotsAlignment() const OVERRIDE { return vixl::kXRegSizeInBytes; }
+  uint32_t GetPreferredSlotsAlignment() const OVERRIDE { return vixl::aarch64::kXRegSizeInBytes; }
 
   JumpTableARM64* CreateJumpTable(HPackedSwitch* switch_instr) {
     jump_tables_.emplace_back(new (GetGraph()->GetArena()) JumpTableARM64(switch_instr));