Utilities/beep: Restrict possible capabilities

We can now just unveil the /dev/beep device node, as well as to restrict
the utility functionality for rpath, wpath & stdio related syscalls only
because we don't actually need anything else.

Userland/Utilities/beep.cpp

@@ -10,6 +10,9 @@
 
 ErrorOr<int> serenity_main(Main::Arguments arguments)
 {
+    TRY(Core::System::pledge("stdio rpath wpath"));
+    TRY(Core::System::unveil("/dev/beep", "rw"));
+    TRY(Core::System::unveil(nullptr, nullptr));
     Optional<size_t> tone;
     Optional<size_t> milliseconds_duration;
     Core::ArgsParser args_parser;