Add *.slnx to UnrealEngine.gitignore

UnrealBuildTool (UBT) auto-generates .slnx solution files the same
way it generates .sln/.vcxproj, based on the project's .uproject and
.Build.cs configuration. These generated files may contain
locally-mounted paths or personal dev environment settings, which can
cause conflicts when shared between team members. As with other
generated project files, they can be regenerated anytime via
'Generate Visual Studio project files' as long as the Source folder,
Content folder, and .uproject file are present.

Docs:
- https://dev.epicgames.com/documentation/en-us/unreal-engine/project-files-for-ides-in-unreal-engine
- https://dev.epicgames.com/documentation/en-us/unreal-engine/unreal-build-tool-in-unreal-engine

UnrealEngine.gitignore

@@ -35,6 +35,7 @@
 *.xcodeproj
 *.xcworkspace
 *.sln
+*.slnx
 *.suo
 *.opensdf
 *.sdf