Include limits.h where needed

This will be needed when TF-PSA-Crypto's `build_info.h` stops including
`limits.h`, which it currently does by accident because it includes
`check_config.h` which wants `limits.h` to check `CHAR_BIT`.

Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>
diff --git a/library/x509.c b/library/x509.c
index f315821..03ca1b7 100644
--- a/library/x509.c
+++ b/library/x509.c
@@ -24,6 +24,7 @@
 #include "mbedtls/oid.h"
 #include "x509_oid.h"
 
+#include <limits.h>
 #include <stdio.h>
 #include <string.h>