file_sys/seed_db: Add missing include guard

Prevents any potential inclusion errors from occurring.

src/core/file_sys/seed_db.h

@@ -2,6 +2,8 @@
 // Licensed under GPLv2 or any later version
 // Refer to the license.txt file included.
 
+#pragma once
+
 #include <array>
 #include <optional>
 #include <vector>