Added the require love.filesystem call to thread.lua

When I ran the base project, I was getting the following error:

```
THREAD ERROR (Thread): libs/inputman/thread.lua:5: module 'vendor.dkjson' not found:
	no field package.preload['vendor.dkjson']
	no file './vendor/dkjson.lua'
	no file '/usr/share/luajit-2.0.2/vendor/dkjson.lua'
	no file '/usr/local/share/lua/5.1/vendor/dkjson.lua'
	no file '/usr/local/share/lua/5.1/vendor/dkjson/init.lua'
	no file '/usr/share/lua/5.1/vendor/dkjson.lua'
	no file '/usr/share/lua/5.1/vendor/dkjson/init.lua'
	no file './vendor/dkjson.so'
	no file '/usr/local/lib/lua/5.1/vendor/dkjson.so'
	no file '/usr/lib/x86_64-linux-gnu/lua/5.1/vendor/dkjson.so'
	no file '/usr/lib/lua/5.1/vendor/dkjson.so'
	no file '/usr/local/lib/lua/5.1/loadall.so'
	no file './vendor.so'
	no file '/usr/local/lib/lua/5.1/vendor.so'
	no file '/usr/lib/x86_64-linux-gnu/lua/5.1/vendor.so'
	no file '/usr/lib/lua/5.1/vendor.so'
	no file '/usr/local/lib/lua/5.1/loadall.so'
```

This was resolved by adding `require("love.filesystem")` to `libs/inputman/thread.lua`.