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_tls.c b/library/ssl_tls.c
index 2a62420..a5562e6 100644
--- a/library/ssl_tls.c
+++ b/library/ssl_tls.c
@@ -41,6 +41,7 @@
 #include <string.h>
 
 #if defined(MBEDTLS_USE_PSA_CRYPTO)
+#include "md_psa.h"
 #include "mbedtls/psa_util.h"
 #include "psa/crypto.h"
 #endif