Avoid performance warning in MSVC compilation.

https://msdn.microsoft.com/en-us/library/windows/desktop/bb773674.aspx says PathIsRootW() returns TRUE or FALSE.

src/filesystem_boost.cpp

@@ -981,7 +981,7 @@ bool is_root(const std::string& path)
 	// See also: <https://googleprojectzero.blogspot.com/2016/02/the-definitive-guide-on-win32-to-nt.html>
 	//
 	const std::wstring& wpath = bfs::path{path}.make_preferred().wstring();
-	return PathIsRootW(wpath.c_str());
+	return PathIsRootW(wpath.c_str()) == TRUE;
 #endif
 }