[benchmark] Mention another cherry-picked change in README.LLVM. NFC.

This was cherry-picked in SVN r342450.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@342506 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/utils/benchmark/README.LLVM b/utils/benchmark/README.LLVM
index 332632a..b425ec7 100644
--- a/utils/benchmark/README.LLVM
+++ b/utils/benchmark/README.LLVM
@@ -15,3 +15,5 @@
   is applied to comply with the LLVM's required CMake version
 * https://github.com/google/benchmark/commit/f0901417c89d123474e6b91365029cfe32cf89dc
   is applied to fix 32-bit build failure on macOS
+* https://github.com/google/benchmark/commit/52613079824ac58d06c070aa9fbbb186a5859e2c
+  is applied to fix cross compilation with MinGW headers