Use forked tarantool/curl repository

A manual action is needed after pulling of this commit to actually use
the downstream repository instead of the upstream one:

sed -i -e 's@https://github.com/curl/curl.git@https://github.com/tarantool/curl.git@' .git/config

It is part of our processes to use forked repositories for submodules.

Suggested by Konstantin Osipov.

.gitmodules

@@ -39,4 +39,4 @@
 	url = https://github.com/tarantool/serpent.git
 [submodule "third_party/curl"]
 	path = third_party/curl
-	url = https://github.com/curl/curl.git
+	url = https://github.com/tarantool/curl.git