Fix build tags of loadavg collector

The collector is only implemented for a subset of all operating systems
supported by go. Compilation will fail if attempted for another OS
target.

collector/loadavg.go

@@ -11,6 +11,7 @@
 // See the License for the specific language governing permissions and
 // limitations under the License.
 
+// +build darwin dragonfly freebsd linux netbsd openbsd solaris
 // +build !noloadavg
 
 package collector

collector/loadavg_freebsd.go

@@ -1,3 +1,16 @@
+// Copyright 2016 The Prometheus 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.
+
 // +build !noloadavg
 
 package collector

collector/loadavg_solaris.go

@@ -11,7 +11,6 @@
 // See the License for the specific language governing permissions and
 // limitations under the License.
 
-// +build solaris
 // +build !noloadavg
 
 package collector