Clean up a few references to System/. We still have docs/SystemLibrary.html
lying around...

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@141703 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/lib/Support/Mutex.cpp b/lib/Support/Mutex.cpp
index b408973..8874e94 100644
--- a/lib/Support/Mutex.cpp
+++ b/lib/Support/Mutex.cpp
@@ -152,6 +152,6 @@
 #elif defined( LLVM_ON_WIN32)
 #include "Windows/Mutex.inc"
 #else
-#warning Neither LLVM_ON_UNIX nor LLVM_ON_WIN32 was set in System/Mutex.cpp
+#warning Neither LLVM_ON_UNIX nor LLVM_ON_WIN32 was set in Support/Mutex.cpp
 #endif
 #endif