pk: move mbedtls_pk_load_file to pk_internal

Signed-off-by: Valerio Setti <valerio.setti@nordicsemi.no>
diff --git a/tests/suites/test_suite_pkwrite.function b/tests/suites/test_suite_pkwrite.function
index 733909e..c760090 100644
--- a/tests/suites/test_suite_pkwrite.function
+++ b/tests/suites/test_suite_pkwrite.function
@@ -1,5 +1,5 @@
 /* BEGIN_HEADER */
-#include "mbedtls/pk.h"
+#include "pk_internal.h"
 #include "mbedtls/pem.h"
 #include "mbedtls/oid.h"
 #include "psa/crypto_sizes.h"