Add missing include

Fix build failures with config full

Signed-off-by: Manuel Pégourié-Gonnard <manuel.pegourie-gonnard@arm.com>
diff --git a/library/ssl_tls13_server.c b/library/ssl_tls13_server.c
index 60ffd26..cf61191 100644
--- a/library/ssl_tls13_server.c
+++ b/library/ssl_tls13_server.c
@@ -25,6 +25,7 @@
 #include "mbedtls/error.h"
 #include "mbedtls/platform.h"
 #include "mbedtls/constant_time.h"
+#include "md_psa.h"
 
 #include "ssl_misc.h"
 #include "ssl_tls13_keys.h"