Take the other include include-what-you-use asks for

The peer that counts pending removals names the unordered_map the client keeps them in, and all three iwyu presets ask for it. The ostream one before this was the same shape on the other file.

Signed-off-by: thc1006 <84045975+thc1006@users.noreply.github.com>

Author: thc1006

ext/test/http/curl_http_test.cc

@@ -23,6 +23,7 @@
 #include <sstream>
 #include <string>
 #include <thread>
+#include <unordered_map>
 #include <utility>
 #include <vector>