Internalise functions flagged as internal

Signed-off-by: Thomas Daubney <thomas.daubney@arm.com>
diff --git a/tf-psa-crypto/drivers/builtin/src/sha256.c b/tf-psa-crypto/drivers/builtin/src/sha256.c
index 1bfb701..6566d5f 100644
--- a/tf-psa-crypto/drivers/builtin/src/sha256.c
+++ b/tf-psa-crypto/drivers/builtin/src/sha256.c
@@ -617,7 +617,7 @@
     }
 }
 
-int mbedtls_internal_sha256_process(mbedtls_sha256_context *ctx,
+static int mbedtls_internal_sha256_process(mbedtls_sha256_context *ctx,
                                     const unsigned char data[SHA256_BLOCK_SIZE])
 {
     if (mbedtls_a64_crypto_sha256_has_support()) {