Fix failing MSVC build on GitHub Actions (#80, thanks @striezel)

Author: striezel

test/expected.t.cpp

@@ -9,6 +9,7 @@
 //   A proposal to add a utility class to represent expected monad
 //   by Vicente J. Botet Escriba and Pierre Talbot, http:://wg21.link/p0323
 
+#include <cassert>
 #include "expected-main.t.hpp"
 
 #ifndef nsel_CONFIG_CONFIRMS_COMPILATION_ERRORS