romfs_factory: Remove unnecessary includes and use forward declarations where applicable

Avoids the need to rebuild whatever includes the romfs factory header if
the loader header ever changes. We also don't need to include the main
core header. We can instead include the headers we specifically need.

src/core/file_sys/nca_metadata.h

@@ -7,6 +7,7 @@
 #include <cstring>
 #include <memory>
 #include <vector>
+#include "common/common_funcs.h"
 #include "common/common_types.h"
 #include "common/swap.h"
 #include "core/file_sys/vfs.h"