Quick build fix.

llvm-svn: 210838
diff --git a/clang-tools-extra/modularize/Modularize.cpp b/clang-tools-extra/modularize/Modularize.cpp
index 5dc11b2..4662a7f 100644
--- a/clang-tools-extra/modularize/Modularize.cpp
+++ b/clang-tools-extra/modularize/Modularize.cpp
@@ -175,6 +175,7 @@
 using namespace llvm;
 using namespace llvm::opt;
 using namespace Modularize;
+using std::error_code;
 
 // Option to specify a file name for a list of header files to check.
 cl::opt<std::string>