libgo: don't include <linux/fs.h> when building gen-sysinfo.go

Removing this doesn't change anything at least with glibc 2.33.
The include was added in https://go.dev/cl/6100049 but it's not
clear why.

Fixes PR go/106266

Reviewed-on: https://go-review.googlesource.com/c/gofrontend/+/417294