ART: Fix up small header includes

Test: m
Change-Id: I6978d6eb4b95a6ee810e5a48ca6f5d6c590d4ce1
diff --git a/compiler/optimizing/code_generator_vector_arm64.cc b/compiler/optimizing/code_generator_vector_arm64.cc
index f422b9f..9095ecd 100644
--- a/compiler/optimizing/code_generator_vector_arm64.cc
+++ b/compiler/optimizing/code_generator_vector_arm64.cc
@@ -15,7 +15,9 @@
  */
 
 #include "code_generator_arm64.h"
+
 #include "mirror/array-inl.h"
+#include "mirror/string.h"
 
 using namespace vixl::aarch64;  // NOLINT(build/namespaces)