bcat: Make ProgressServiceBackend's GetEvent() const

This member function doesn't modify internal member state, so it can be
marked const.

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

@@ -16,7 +16,7 @@ ProgressServiceBackend::ProgressServiceBackend(std::string event_name) : impl{}
         kernel, Kernel::ResetType::Automatic, "ProgressServiceBackend:UpdateEvent:" + event_name);
 }
 
-Kernel::SharedPtr<Kernel::ReadableEvent> ProgressServiceBackend::GetEvent() {
+Kernel::SharedPtr<Kernel::ReadableEvent> ProgressServiceBackend::GetEvent() const {
     return event.readable;
 }
 

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

@@ -87,7 +87,7 @@ public:
 private:
     explicit ProgressServiceBackend(std::string event_name);
 
-    Kernel::SharedPtr<Kernel::ReadableEvent> GetEvent();
+    Kernel::SharedPtr<Kernel::ReadableEvent> GetEvent() const;
     DeliveryCacheProgressImpl& GetImpl();
 
     void SignalUpdate() const;