Fix build broken by LLVM commit r207593

llvm-svn: 207685

Author: tstellarAMD

libclc/utils/prepare-builtins.cpp

@@ -7,6 +7,7 @@
 #include "llvm/Support/CommandLine.h"
 #include "llvm/Support/ManagedStatic.h"
 #include "llvm/Support/MemoryBuffer.h"
+#include "llvm/Support/FileSystem.h"
 #include "llvm/Support/raw_ostream.h"
 #include "llvm/Support/system_error.h"
 #include "llvm/Support/ErrorOr.h"