Update unittests for include change.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@159724 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/unittests/Frontend/FrontendActionTest.cpp b/unittests/Frontend/FrontendActionTest.cpp
index 2d4befc..84a6545 100644
--- a/unittests/Frontend/FrontendActionTest.cpp
+++ b/unittests/Frontend/FrontendActionTest.cpp
@@ -8,6 +8,7 @@
 //===----------------------------------------------------------------------===//
 
 #include "clang/AST/RecursiveASTVisitor.h"
+#include "clang/AST/ASTContext.h"
 #include "clang/AST/ASTConsumer.h"
 #include "clang/Frontend/CompilerInstance.h"
 #include "clang/Frontend/CompilerInvocation.h"