remove warnings from google-benchmarks in libcxx

Differential Revision: https://reviews.llvm.org/D25522

Patch written by Aditya Kumar.

llvm-svn: 284179
Cr-Mirrored-From: sso://chromium.googlesource.com/_direct/external/github.com/llvm/llvm-project
Cr-Mirrored-Commit: 622c26389ffa6fd2dfb580289cce63f3e48a658c
diff --git a/benchmarks/GenerateInput.hpp b/benchmarks/GenerateInput.hpp
index 49fb48d..affd541 100644
--- a/benchmarks/GenerateInput.hpp
+++ b/benchmarks/GenerateInput.hpp
@@ -112,7 +112,7 @@
 
 inline std::vector<std::string> getRandomStringInputs(size_t N) {
     std::vector<std::string> inputs;
-    for (int i=0; i < N; ++i) {
+    for (size_t i=0; i < N; ++i) {
         inputs.push_back(getRandomString(1024));
     }
     return inputs;