Fix an error from MSVC.

Change-Id: I32b5e032f9c0c0da6075922afe99bfd864c1f166
Reviewed-on: https://code-review.googlesource.com/5246
Reviewed-by: Paul Wankadia <junyer@google.com>
diff --git a/util/bitmap.h b/util/bitmap.h
index 2e95c85..36106b0 100644
--- a/util/bitmap.h
+++ b/util/bitmap.h
@@ -46,9 +46,9 @@
 #if defined(__GNUC__)
     return __builtin_ctzll(n);
 #elif defined(_MSC_VER)
-    int c;
+    unsigned long c;
     _BitScanForward64(&c, n);
-    return c;
+    return static_cast<int>(c);
 #else
 #error "bit scan forward not implemented"
 #endif