bcat: In-class initialize ProgressServiceBackend's impl member

Allows us to remove a constructor initializer list.

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

@@ -10,7 +10,7 @@
 
 namespace Service::BCAT {
 
-ProgressServiceBackend::ProgressServiceBackend(std::string_view event_name) : impl{} {
+ProgressServiceBackend::ProgressServiceBackend(std::string_view event_name) {
     auto& kernel{Core::System::GetInstance().Kernel()};
     event = Kernel::WritableEvent::CreateEventPair(
         kernel, Kernel::ResetType::Automatic,

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

@@ -95,7 +95,7 @@ private:
 
     void SignalUpdate() const;
 
-    DeliveryCacheProgressImpl impl;
+    DeliveryCacheProgressImpl impl{};
     Kernel::EventPair event;
     bool need_hle_lock = false;
 };