Fix unused function when MBEDTLS_AES_SETKEY_ENC_ALT set

Signed-off-by: Dave Rodgman <dave.rodgman@arm.com>
diff --git a/library/aes.c b/library/aes.c
index 0a61d1b..2173611 100644
--- a/library/aes.c
+++ b/library/aes.c
@@ -348,6 +348,7 @@
 static uint32_t RT3[256];
 #endif /* !MBEDTLS_AES_FEWER_TABLES */
 
+#if !defined(MBEDTLS_AES_SETKEY_ENC_ALT)
 /*
  * Round constants
  */
@@ -438,6 +439,8 @@
     }
 }
 
+#endif /* !defined(MBEDTLS_AES_SETKEY_ENC_ALT) */
+
 #undef ROTL8
 
 #endif /* MBEDTLS_AES_ROM_TABLES */