Make cgroup.{cc,hh} linux-only files

Forcing a conditional include, vs making the headers content
conditional, I think is more maintainable.

It is also how the other platform-specific headers (like
`namespaces.hh`) have been adapted.

src/libstore/build/local-derivation-goal.cc

@@ -14,7 +14,6 @@
 #include "topo-sort.hh"
 #include "callback.hh"
 #include "json-utils.hh"
-#include "cgroup.hh"
 #include "personality.hh"
 #include "current-process.hh"
 #include "child.hh"
@@ -52,6 +51,7 @@
 #   include <seccomp.h>
 # endif
 # define pivot_root(new_root, put_old) (syscall(SYS_pivot_root, new_root, put_old))
+# include "cgroup.hh"
 #endif
 
 #if __APPLE__

src/libutil/linux/cgroup.cc

@@ -1,5 +1,3 @@
-#if __linux__
-
 #include "cgroup.hh"
 #include "util.hh"
 #include "file-system.hh"
@@ -145,5 +143,3 @@ CgroupStats destroyCgroup(const Path & cgroup)
 }
 
 }
-
-#endif

src/libutil/linux/cgroup.hh

@@ -1,8 +1,6 @@
 #pragma once
 ///@file
 
-#if __linux__
-
 #include <chrono>
 #include <optional>
 
@@ -28,5 +26,3 @@ struct CgroupStats
 CgroupStats destroyCgroup(const Path & cgroup);
 
 }
-
-#endif