file_sys: Replace includes with forward declarations where applicable

Cuts down on include dependencies, resulting in less files that need to
be rebuilt when certain things are changed.

src/core/file_sys/bis_factory.cpp

@@ -3,6 +3,7 @@
 // Refer to the license.txt file included.
 
 #include "core/file_sys/bis_factory.h"
+#include "core/file_sys/registered_cache.h"
 
 namespace FileSys {
 
@@ -13,6 +14,8 @@ BISFactory::BISFactory(VirtualDir nand_root_)
       usrnand_cache(std::make_shared<RegisteredCache>(
           GetOrCreateDirectoryRelative(nand_root, "/user/Contents/registered"))) {}
 
+BISFactory::~BISFactory() = default;
+
 std::shared_ptr<RegisteredCache> BISFactory::GetSystemNANDContents() const {
     return sysnand_cache;
 }