Fix Clang warning about tautological compare.

src/utils.h

Index: src/utils.h
diff --git a/src/utils.h b/src/utils.h
index dc3a171c8db3e508a93d18707e2d36f4919f2762..e659de22018c434a1d50550ee677deb08544dc9a 100644
--- a/src/utils.h
+++ b/src/utils.h
@@ -974,7 +974,7 @@ class EnumSet {
   T Mask(E element) const {
     // The strange typing in ASSERT is necessary to avoid stupid warnings, see:
     // http://gcc.gnu.org/bugzilla/show_bug.cgi?id=43680
-    ASSERT(element < static_cast<int>(sizeof(T) * CHAR_BIT));
+    ASSERT(static_cast<int>(element) < static_cast<int>(sizeof(T) * CHAR_BIT));
     return 1 << element;
   }