Define OPENSSL_UNUSED for __clang__ as well.

Windows Clang needs this in the stack case too, but it doesn't define
__GNUC__ since it's emulating MSVC.

Change-Id: I646550ca95240e80822adddc2b53c3b58c2ec4a6
Reviewed-on: https://boringssl-review.googlesource.com/16644
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>

include/openssl/base.h

diff --git a/include/openssl/base.h b/include/openssl/base.h
index 8162db3..f00c563 100644
--- a/include/openssl/base.h
+++ b/include/openssl/base.h
@@ -197,7 +197,7 @@
 #define OPENSSL_MSVC_PRAGMA(arg)
 #endif
 
-#if defined(__GNUC__)
+#if defined(__GNUC__) || defined(__clang__)
 #define OPENSSL_UNUSED __attribute__((unused))
 #else
 #define OPENSSL_UNUSED