* Prevent system_linux.go from compiling for android

The problem is described here https://groups.google.com/g/golang-nuts/c/ZTGMjhEV7uY?pli=1
When GOOS=android it compiles both *_android.go and *_linux.go files, so you need to explicitly disable the file for android, because android SDL headers do not have those linux functions

* Style fix