Add missing include guards

These were being used in multiple files all throughout the codebase!

src/openrct2/core/BackgroundWorker.hpp

@@ -7,6 +7,8 @@
  * OpenRCT2 is licensed under the GNU General Public License version 3.
  *****************************************************************************/
 
+#pragma once
+
 #include <algorithm>
 #include <atomic>
 #include <condition_variable>

src/openrct2/peep/PeepSpriteIds.h

@@ -7,6 +7,8 @@
  * OpenRCT2 is licensed under the GNU General Public License version 3.
  *****************************************************************************/
 
+#pragma once
+
 namespace OpenRCT2
 {
     enum

src/openrct2/world/map_generator/TreePlacement.h

@@ -7,6 +7,8 @@
  * OpenRCT2 is licensed under the GNU General Public License version 3.
  *****************************************************************************/
 
+#pragma once
+
 namespace OpenRCT2::World::MapGenerator
 {
     struct Settings;

test/tests/TestData.h

@@ -7,10 +7,10 @@
  * OpenRCT2 is licensed under the GNU General Public License version 3.
  *****************************************************************************/
 
-#include <string>
-
 #pragma once
 
+#include <string>
+
 namespace TestData
 {
     std::string GetBasePath();

test/tests/helpers/StringHelpers.hpp

@@ -7,6 +7,8 @@
  * OpenRCT2 is licensed under the GNU General Public License version 3.
  *****************************************************************************/
 
+#pragma once
+
 #include <cassert>
 #include <string>
 #include <string_view>