[mbedtls] update mbedtls-config.h to fix build (#3338)

- Add stdio.h for snprintf.

- When `MBEDTLS_ECP_ALT` is defined, need to define mbedtls_ecp_restart_ctx
for build to succeed.

third_party/mbedtls/mbedtls-config.h

@@ -29,6 +29,7 @@
 #ifndef MBEDTLS_CONFIG_H
 #define MBEDTLS_CONFIG_H
 
+#include <stdio.h>
 #include <stdlib.h>
 
 #include <openthread/config.h>
@@ -167,6 +168,10 @@ __inline void mbedtls_platform_zeroize( void *buf, size_t len)
 #include MBEDTLS_USER_CONFIG_FILE
 #endif
 
+#if defined(MBEDTLS_ECP_ALT) && !defined(MBEDTLS_ECP_RESTARTABLE)
+typedef void mbedtls_ecp_restart_ctx;
+#endif
+
 #include "mbedtls/check_config.h"
 
 #endif /* MBEDTLS_CONFIG_H */