style: add braces around single-statement bodies

- wrap single-statement control-flow bodies in braces
- clang-tidy readability-braces-around-statements
  https://clang.llvm.org/extra/clang-tidy/checks/readability/braces-around-statements.html

fairmq/PluginServices.cxx

@@ -17,7 +17,9 @@ auto PluginServices::ChangeDeviceState(const string& controller, const DeviceSta
 {
     lock_guard<mutex> lock{fDeviceControllerMutex};
 
-    if (!fDeviceController) fDeviceController = controller;
+    if (!fDeviceController) {
+        fDeviceController = controller;
+    }
 
     if (fDeviceController == controller) {
         return fDevice.ChangeState(next);

test/event_manager/_event_manager.cxx

@@ -29,13 +29,17 @@ TEST(EventManager, Basics)
     std::function<void(typename TestEvent::KeyType, int)> callback{
         [&](TestEvent::KeyType key, int newValue){
             ++call_counter;
-            if (key == "test") value = newValue;
+            if (key == "test") {
+                value = newValue;
+            }
         }
     };
     std::function<void(typename TestEvent::KeyType, string)> callback2{
         [&](TestEvent::KeyType key, string newValue){
             ++call_counter2;
-            if (key == "test") value2 = newValue;
+            if (key == "test") {
+                value2 = newValue;
+            }
         }
     };