Add StringRef include to libc benchmark.

GitOrigin-RevId: f1ecf560bf70d67b6ffdb800c9d33390fd8b249c
diff --git a/utils/benchmarks/LibcBenchmark.cpp b/utils/benchmarks/LibcBenchmark.cpp
index b1ab79e..cef595d 100644
--- a/utils/benchmarks/LibcBenchmark.cpp
+++ b/utils/benchmarks/LibcBenchmark.cpp
@@ -7,6 +7,7 @@
 //===----------------------------------------------------------------------===//
 
 #include "LibcBenchmark.h"
+#include "llvm/ADT/StringRef.h"
 #include "llvm/Support/Host.h"
 
 namespace llvm {