[mlir] Fix the build

This patch fixes:

  llvm/include/llvm/ADT/SmallVector.h:1162:7: error: implicit
  instantiation of undefined template 'std::basic_string<char>'

This happens under -DCMAKE_CXX_FLAGS="-stdlib=libc++".

Author: kazutakahirata

mlir/include/mlir/Conversion/ConvertToEmitC/ConvertToEmitCPass.h

@@ -12,6 +12,7 @@
 #include "llvm/ADT/SmallVector.h"
 
 #include <memory>
+#include <string>
 
 namespace mlir {
 class Pass;