crypto: move legacy symbols support to a dedicated header file

Signed-off-by: Valerio Setti <valerio.setti@nordicsemi.no>
diff --git a/include/mbedtls/config_psa.h b/include/mbedtls/config_psa.h
index 9a86b61..c30ba45 100644
--- a/include/mbedtls/config_psa.h
+++ b/include/mbedtls/config_psa.h
@@ -42,6 +42,8 @@
 #include MBEDTLS_PSA_CRYPTO_USER_CONFIG_FILE
 #endif
 
+#include "psa/crypto_legacy.h"
+
 #ifdef __cplusplus
 extern "C" {
 #endif