bootmanager: In-class initialize EmuThread's boolean member variables

Trims down the initializer list a little.

src/citra_qt/bootmanager.h

@@ -71,9 +71,9 @@ public:
     };
 
 private:
-    bool exec_step;
-    bool running;
-    std::atomic<bool> stop_run;
+    bool exec_step = false;
+    bool running = false;
+    std::atomic<bool> stop_run{false};
     std::mutex running_mutex;
     std::condition_variable running_cv;