go.mod: bump golang.org/x/sys from 0.14.0 to 0.15.0

Bumps [golang.org/x/sys](https://github.com/golang/sys) from 0.14.0 to 0.15.0.
- [Commits](golang/sys@v0.14.0...v0.15.0)

---
updated-dependencies:
- dependency-name: golang.org/x/sys
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

Signed-off-by: dependabot[bot] <support@github.com>

go.mod

@@ -2,4 +2,4 @@ module github.com/tklauser/numcpus
 
 go 1.13
 
-require golang.org/x/sys v0.14.0
+require golang.org/x/sys v0.15.0

go.sum

@@ -1,2 +1,2 @@
-golang.org/x/sys v0.14.0 h1:Vz7Qs629MkJkGyHxUlRHizWJRG2j8fbQKjELVSNhy7Q=
-golang.org/x/sys v0.14.0/go.mod h1:/VUhepiaJMQUp4+oa/7Zr1D23ma6VTLIYjOOTFZPUcA=
+golang.org/x/sys v0.15.0 h1:h48lPFYpsTvQJZF4EKyI4aLHaev3CxivZmv7yZig9pc=
+golang.org/x/sys v0.15.0/go.mod h1:/VUhepiaJMQUp4+oa/7Zr1D23ma6VTLIYjOOTFZPUcA=