Use build tags to make it compile on non linux platforms

Signed-off-by: Sylvain Rabot <sylvain@abstraction.fr>

internal/runtime/cpu_linux.go

@@ -1,3 +1,5 @@
+// +build linux
+
 /*
 Copyright 2018 The Kubernetes Authors.
 

internal/runtime/cpu_notlinux.go

@@ -0,0 +1,28 @@
+// +build !linux
+
+/*
+Copyright 2018 The Kubernetes Authors.
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+*/
+
+package runtime
+
+import (
+	"runtime"
+)
+
+// NumCPU ...
+func NumCPU() int {
+	return runtime.NumCPU()
+}