Create psa_util_internal.h

Most functions in psa_util.h are going to end up there (except those
that can be static in one file), but I wanted to have separate commits
for file creation and moving code around, so for now the new file's
pretty empty but that will change in the next few commits.

Signed-off-by: Manuel Pégourié-Gonnard <manuel.pegourie-gonnard@arm.com>
diff --git a/library/ssl_tls12_client.c b/library/ssl_tls12_client.c
index e4f3821..8aaf37a 100644
--- a/library/ssl_tls12_client.c
+++ b/library/ssl_tls12_client.c
@@ -31,7 +31,7 @@
 #include "mbedtls/constant_time.h"
 
 #if defined(MBEDTLS_USE_PSA_CRYPTO)
-#include "mbedtls/psa_util.h"
+#include "psa_util_internal.h"
 #include "psa/crypto.h"
 #if defined(MBEDTLS_KEY_EXCHANGE_ECDHE_PSK_ENABLED)
 /* Define a local translating function to save code size by not using too many