Internalise functions flagged as internal

Signed-off-by: Thomas Daubney <thomas.daubney@arm.com>
diff --git a/tf-psa-crypto/drivers/builtin/src/md5.c b/tf-psa-crypto/drivers/builtin/src/md5.c
index 98b8ee4..519be43 100644
--- a/tf-psa-crypto/drivers/builtin/src/md5.c
+++ b/tf-psa-crypto/drivers/builtin/src/md5.c
@@ -58,7 +58,7 @@
     return 0;
 }
 
-int mbedtls_internal_md5_process(mbedtls_md5_context *ctx,
+static int mbedtls_internal_md5_process(mbedtls_md5_context *ctx,
                                  const unsigned char data[64])
 {
     struct {