Implement and use MBEDTLS_STATIC_ASSERT()

Fixes #3693

Signed-off-by: Tom Cosgrove <tom.cosgrove@arm.com>
diff --git a/library/ssl_tls12_server.c b/library/ssl_tls12_server.c
index 0806f7f..631331d 100644
--- a/library/ssl_tls12_server.c
+++ b/library/ssl_tls12_server.c
@@ -1510,10 +1510,9 @@
             MBEDTLS_TLS_SIG_NONE
         };
 
-#if defined(static_assert)
-        static_assert(sizeof(default_sig_algs) / sizeof(default_sig_algs[0]) <=
-                      MBEDTLS_RECEIVED_SIG_ALGS_SIZE, "default_sig_algs is too big");
-#endif
+        MBEDTLS_STATIC_ASSERT(sizeof(default_sig_algs) / sizeof(default_sig_algs[0])
+                              <= MBEDTLS_RECEIVED_SIG_ALGS_SIZE,
+                              "default_sig_algs is too big");
 
         memcpy(received_sig_algs, default_sig_algs, sizeof(default_sig_algs));
     }