RFR: Fix aarch64 build failure

[Aleksey Shipilev]

Missed this from recent build fixes:

diff -r 040b0d8b5f24 src/hotspot/cpu/aarch64/gc/shenandoah/shenandoahBarrierSetAssembler_aarch64.cpp
--- a/src/hotspot/cpu/aarch64/gc/shenandoah/shenandoahBarrierSetAssembler_aarch64.cpp	Thu Apr 12
14:07:31 2018 -0400
+++ b/src/hotspot/cpu/aarch64/gc/shenandoah/shenandoahBarrierSetAssembler_aarch64.cpp	Fri Apr 13
10:55:33 2018 +0200
@@ -22,6 +22,7 @@
  */

 #include "precompiled.hpp"
+#include "gc/shenandoah/shenandoahBarrierSet.hpp"
 #include "gc/shenandoah/shenandoahBarrierSetAssembler.hpp"

 #define __ masm->

Thanks,
-Aleksey