Add missing include in missile_test.cpp

[ephphatha]

prevented building tests on macos

[AJenbo]

/root/repo/Source/pfile.cpp:366:61: error: invalid initialization of reference of type 'const wstring& {aka const std::__cxx11::basic_string<wchar_t>&}' from expression of type 'const std::__cxx11::basic_string' size_t bytes = static_cast<size_t>(std::stoi(bitsAsString) / 8);

Hm hopefully we can upgrade GCC for Amiga soon

[ephphatha]

welp :D

Also turns out it builds without this include after installing xcode extras, must've been using an outdated toolchain previously... probably still worth it given the use of that class is unrelated to any of the other headers included in missile_test.cpp