Kernel: Fix -Wunreachable-code warnings from clang

Author: https://github.com/nico
Commit: 1cdb12e920
Pull-request: https://github.com/SerenityOS/serenity/pull/10399

Kernel/Syscalls/prctl.cpp

@@ -18,10 +18,8 @@ KResultOr<FlatPtr> Process::sys$prctl(int option, FlatPtr arg1, [[maybe_unused]]
     case PR_SET_DUMPABLE:
         set_dumpable(arg1);
         return 0;
-    default:
-        return EINVAL;
     }
-    return 0;
+    return EINVAL;
 }
 
 }