Linux: Flush stdout explicitly when reading stdin (#1172)

Rules of automatic flushing of stdout buffer is implementation-defined
behaviour. In glibc this is automatically flushed, but we can't rely
on it for other implementations such as musl.

src/Main/TextUserInterface.cpp

@@ -1047,7 +1047,7 @@ namespace VeraCrypt
 
 	void TextUserInterface::DoShowString (const wxString &str) const
 	{
-		wcout << str.c_str();
+		wcout << str.c_str() << flush;
 	}
 
 	void TextUserInterface::DoShowWarning (const wxString &message) const