openssl: use colons properly in the ciphers list

While the previous string worked, this is the documented format.

Reported-by: Richard Moore

lib/vtls/openssl.h

@@ -107,7 +107,7 @@ bool Curl_ossl_cert_status_request(void);
 #define curlssl_cert_status_request() Curl_ossl_cert_status_request()
 
 #define DEFAULT_CIPHER_SELECTION \
-  "ALL!EXPORT!EXPORT40!EXPORT56!aNULL!LOW!RC4@STRENGTH"
+  "ALL:!EXPORT:!EXPORT40:!EXPORT56:!aNULL:!LOW:!RC4:@STRENGTH"
 
 #endif /* USE_OPENSSL */
 #endif /* HEADER_CURL_SSLUSE_H */