Split build_info.h: create mbedtls/config_adjust_x509.h

There isn't anything to put in this file. Create it anyway for consistency
with crypto and TLS.

Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>
diff --git a/include/mbedtls/build_info.h b/include/mbedtls/build_info.h
index 510e1a5..872e841 100644
--- a/include/mbedtls/build_info.h
+++ b/include/mbedtls/build_info.h
@@ -122,6 +122,8 @@
 
 #include "mbedtls/config_adjust_legacy_crypto.h"
 
+#include "mbedtls/config_adjust_x509.h"
+
 /* The following blocks make it easier to disable all of TLS,
  * or of TLS 1.2 or 1.3 or DTLS, without having to manually disable all
  * key exchanges, options and extensions related to them. */