Remove C4245 suppression.

Chromium builds with this warning on. This lets us notice problems (of
which there were only one) sooner. I'll try to align the other warnings
in a follow-up.

Change-Id: Id0960b782733b799e1c3e82f89c2aaba0bdd6833
Reviewed-on: https://boringssl-review.googlesource.com/21164
Commit-Queue: David Benjamin <davidben@google.com>
Commit-Queue: Steven Valdez <svaldez@google.com>
Reviewed-by: Steven Valdez <svaldez@google.com>
CQ-Verified: CQ bot account: commit-bot@chromium.org <commit-bot@chromium.org>
diff --git a/include/openssl/span.h b/include/openssl/span.h
index 97361c2..3109036 100644
--- a/include/openssl/span.h
+++ b/include/openssl/span.h
@@ -105,7 +105,7 @@
       std::is_convertible<decltype(std::declval<C>().data()), T *>::value &&
       std::is_integral<decltype(std::declval<C>().size())>::value>;
 
-  static const size_t npos = -1;
+  static const size_t npos = static_cast<size_t>(-1);
 
  public:
   constexpr Span() : Span(nullptr, 0) {}