add missing include of <utility> for std::unreachable (#1558)

Author: ericniebler

include/stdexec/__detail/__config.hpp

@@ -40,6 +40,7 @@
 #include <cassert>
 #include <cstdlib>
 #include <type_traits> // IWYU pragma: keep
+#include <utility> // IWYU pragma: keep for std::unreachable
 
 // When used with no arguments, these macros expand to 1 if the current
 // compiler corresponds to the macro name; 0, otherwise. When used with arguments,