Support MSVS with clang

Signed-off-by: Dave Rodgman <dave.rodgman@arm.com>
diff --git a/library/common.h b/library/common.h
index 436e35a..de26d45 100644
--- a/library/common.h
+++ b/library/common.h
@@ -335,7 +335,7 @@
 #endif
 
 /* Suppress compiler warnings for unused functions and variables. */
-#if !defined(MBEDTLS_MAYBE_UNUSED) && defined(__GNUC__)
+#if !defined(MBEDTLS_MAYBE_UNUSED) && (defined(__GNUC__) || defined(__clang__))
 #define MBEDTLS_MAYBE_UNUSED __attribute__((unused))
 #endif
 #if !defined(MBEDTLS_MAYBE_UNUSED) && defined(__IAR_SYSTEMS_ICC__) && defined(__VER__)