Return certificate serial

OpenVPN Adapter/OpenVPNCertificate.m

@@ -9,6 +9,7 @@
 
 #include <mbedtls/x509_crt.h>
 #include <mbedtls/pem.h>
+#include <mbedtls/bignum.h>
 
 #import "NSError+OpenVPNError.h"
 
@@ -52,6 +53,10 @@
     return certificate;
 }
 
+- (NSData *)serial {
+    return [NSData dataWithBytes:self.crt->serial.p length:self.crt->serial.len];
+}
+
 - (instancetype)init
 {
     if (self = [super init]) {