[ASan] fix cmake build warning

git-svn-id: https://llvm.org/svn/llvm-project/compiler-rt/trunk@160957 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/lib/asan/asan_mac.cc b/lib/asan/asan_mac.cc
index 973fe88..a3d39e7 100644
--- a/lib/asan/asan_mac.cc
+++ b/lib/asan/asan_mac.cc
@@ -56,7 +56,7 @@
   int mib[2] = { CTL_KERN, KERN_OSRELEASE };
   char version[100];
   uptr len = 0, maxlen = sizeof(version) / sizeof(version[0]);
-  for (int i = 0; i < maxlen; i++) version[i] = '\0';
+  for (uptr i = 0; i < maxlen; i++) version[i] = '\0';
   // Get the version length.
   CHECK(sysctl(mib, 2, 0, &len, 0, 0) != -1);
   CHECK(len < maxlen);