Build the Cgroup fingerprinter on only Linux.

Change the logic from `!linux` to an empty build tag so that *if*
another platform picks up Cgroups support they can add themselves to the
necessary build tags for this fingerprinter and be on their way.
Because this technology isn't inherently Linux-specific and isn't
mutually exclusive of other resource isolation containers, resist the
urge to rename the Cgroup fingerprinter to something generic like the
ResourceContainerFingerprinter.

client/fingerprint/cgroup.go

@@ -1,3 +1,5 @@
+// +build linux
+
 package fingerprint
 
 import (

client/fingerprint/cgroup_default.go

@@ -1,4 +1,4 @@
-// +build !linux
+// +build
 
 package fingerprint
 

client/fingerprint/cgroup_test.go

@@ -1,3 +1,5 @@
+// +build linux
+
 package fingerprint
 
 import (