Keep SSL context const when hw accel is disabled

Signed-off-by: Manuel Pégourié-Gonnard <manuel.pegourie-gonnard@arm.com>
diff --git a/library/ssl_tls.c b/library/ssl_tls.c
index bfcfcbb..52f56cf 100644
--- a/library/ssl_tls.c
+++ b/library/ssl_tls.c
@@ -1253,7 +1253,10 @@
                                    const unsigned char randbytes[64],
                                    int minor_ver,
                                    unsigned endpoint,
-                                   const mbedtls_ssl_context *ssl )
+#if !defined(MBEDTLS_SSL_HW_RECORD_ACCEL)
+                                   const
+#endif
+                                   mbedtls_ssl_context *ssl )
 {
     int ret = 0;
     unsigned char keyblk[256];