Condition the use of UI64 on _MSC_VER.

Using OPENSSL_WINDOWS for this is inaccurate because it's really a
feature of the compiler, not the platform. I think it's only MSVC that
uses the UI64 suffix.

Change-Id: I4a95961b94e69e72b93f5ed1e0457661b74242c8
Reviewed-on: https://boringssl-review.googlesource.com/2730
Reviewed-by: David Benjamin <davidben@chromium.org>
Reviewed-by: Adam Langley <agl@google.com>

crypto/internal.h

diff --git a/crypto/internal.h b/crypto/internal.h
index 5b980d8..c5890d5 100644
--- a/crypto/internal.h
+++ b/crypto/internal.h
@@ -132,7 +132,7 @@
 };
 
 
-#if defined(OPENSSL_WINDOWS)
+#if defined(_MSC_VER)
 #define OPENSSL_U64(x) x##UI64
 #else
 
@@ -142,7 +142,7 @@
 #define OPENSSL_U64(x) x##ULL
 #endif
 
-#endif  /* OPENSSL_WINDOWS */
+#endif  /* defined(_MSC_VER) */
 
 #if defined(OPENSSL_X86) || defined(OPENSSL_X86_64)
 /* OPENSSL_cpuid_setup initializes OPENSSL_ia32cap_P. */