UI: Hide --portable from help text if disallowed

UI/obs-app.cpp

@@ -3068,7 +3068,9 @@ int main(int argc, char *argv[])
 				"--scene <string>: Start with specific scene.\n\n"
 				"--studio-mode: Enable studio mode.\n"
 				"--minimize-to-tray: Minimize to system tray.\n"
+#if ALLOW_PORTABLE_MODE
 				"--portable, -p: Use portable mode.\n"
+#endif
 				"--multi, -m: Don't warn when launching multiple instances.\n\n"
 				"--verbose: Make log more verbose.\n"
 				"--always-on-top: Start in 'always on top' mode.\n\n"