add missing header

Author: ericniebler

include/stdexec/__detail/__connect_awaitable.hpp

@@ -18,6 +18,7 @@
 #include "__execution_fwd.hpp"
 
 #include "__awaitable.hpp"
+#include "__completion_signatures.hpp"
 #include "__concepts.hpp"
 #include "__config.hpp"
 #include "__env.hpp"