Move mbedtls_to_psa_error declaration to internal header

Signed-off-by: Steven Cooreman <steven.cooreman@silabs.com>
diff --git a/tests/src/drivers/signature.c b/tests/src/drivers/signature.c
index 62ba407..04c5de4 100644
--- a/tests/src/drivers/signature.c
+++ b/tests/src/drivers/signature.c
@@ -25,6 +25,7 @@
 
 #if defined(MBEDTLS_PSA_CRYPTO_DRIVERS) && defined(PSA_CRYPTO_DRIVER_TEST)
 #include "psa/crypto.h"
+#include "psa_crypto_core.h"
 #include "mbedtls/ecp.h"
 
 #include "drivers/signature.h"