Revert "selftest: Remove X.509 selftest"

This reverts commit 47a3635fc7107c7d838816475c6c816d9b47f047.
diff --git a/programs/test/selftest.c b/programs/test/selftest.c
index bde1163..bd28e9a 100644
--- a/programs/test/selftest.c
+++ b/programs/test/selftest.c
@@ -51,6 +51,7 @@
 #include "mbedtls/base64.h"
 #include "mbedtls/bignum.h"
 #include "mbedtls/rsa.h"
+#include "mbedtls/x509.h"
 #include "mbedtls/xtea.h"
 #include "mbedtls/pkcs5.h"
 #include "mbedtls/ecp.h"
@@ -316,6 +317,9 @@
 #if defined(MBEDTLS_RSA_C)
     {"rsa", mbedtls_rsa_self_test},
 #endif
+#if defined(MBEDTLS_X509_USE_C)
+    {"x509", mbedtls_x509_self_test},
+#endif
 #if defined(MBEDTLS_XTEA_C)
     {"xtea", mbedtls_xtea_self_test},
 #endif