Another libc++ warning suppression on Linux; no functionality change

llvm-svn: 174637
Cr-Mirrored-From: sso://chromium.googlesource.com/_direct/external/github.com/llvm/llvm-project
Cr-Mirrored-Commit: 63f700e4d1876abcce74276dbf9838f9902e7664
diff --git a/src/thread.cpp b/src/thread.cpp
index b2bd07e..184d81e 100644
--- a/src/thread.cpp
+++ b/src/thread.cpp
@@ -71,7 +71,7 @@
     // does not have a definite limit.
     if (result == -1)
         return 0;
-    return result;
+    return static_cast<unsigned>(result);
 #else  // defined(CTL_HW) && defined(HW_NCPU)
     // TODO: grovel through /proc or check cpuid on x86 and similar
     // instructions on other architectures.