Changed x509_internal.h methods as static.
Moved some functions under defined to get rid of compiler warnings.
Functions moved under defines:
 - mbedtls_x509_get_alg
 - mbedtls_x509_get_alg_null
 - mbedtls_x509_get_time
 - mbedtls_x509_get_ext
 - mbedtls_x509_sig_alg_gets
 - mbedtls_x509_key_size_helper

Left one function (mbedtls_x509_write_names) as non static as it increased code size.
diff --git a/tests/suites/test_suite_x509write.function b/tests/suites/test_suite_x509write.function
index 9237165..3ff5c5f 100644
--- a/tests/suites/test_suite_x509write.function
+++ b/tests/suites/test_suite_x509write.function
@@ -6,6 +6,10 @@
 #include "mbedtls/pem.h"
 #include "mbedtls/oid.h"
 #include "mbedtls/rsa.h"
+/* We need to include x509.c because we are testing x509 internal
+ * functions from x509_internal.h which are static. With this include
+ * we get the tested functions defined. */
+#include "../library/x509.c"
 
 #if defined(MBEDTLS_RSA_C)
 int mbedtls_rsa_decrypt_func( void *ctx, int mode, size_t *olen,