bcat: Take std::function instance by value in NullBackend's constructor

Without this, the std::move within the constructor initializer list
won't be able to actually perform a move.

src/core/hle/service/bcat/backend/backend.cpp

@@ -96,7 +96,7 @@ Backend::Backend(DirectoryGetter getter) : dir_getter(std::move(getter)) {}
 
 Backend::~Backend() = default;
 
-NullBackend::NullBackend(const DirectoryGetter& getter) : Backend(std::move(getter)) {}
+NullBackend::NullBackend(DirectoryGetter getter) : Backend(std::move(getter)) {}
 
 NullBackend::~NullBackend() = default;
 

src/core/hle/service/bcat/backend/backend.h

@@ -131,7 +131,7 @@ protected:
 // A backend of BCAT that provides no operation.
 class NullBackend : public Backend {
 public:
-    explicit NullBackend(const DirectoryGetter& getter);
+    explicit NullBackend(DirectoryGetter getter);
     ~NullBackend() override;
 
     bool Synchronize(TitleIDVersion title, ProgressServiceBackend& progress) override;