Add missing X509_set_proxy_flag num

Reviewed-by: NRichard Levitte <levitte@openssl.org>
Reviewed-by: NRich Salz <rsalz@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/1343)

util/libcrypto.num

@@ -4152,3 +4152,4 @@ HMAC_CTX_get_md                         4096	1_1_0	EXIST::FUNCTION:
 NAME_CONSTRAINTS_check_CN               4097	1_1_0	EXIST::FUNCTION:
 OCSP_resp_get0_id                       4098	1_1_0	EXIST::FUNCTION:OCSP
 OCSP_resp_get0_certs                    4099	1_1_0	EXIST::FUNCTION:OCSP
+X509_set_proxy_flag                     4100	1_1_0	EXIST::FUNCTION: