fd/atomic: Better separate this orthogonal functionality

Nothing is using the old interface, so we can move it for better
separation of responisbilities.

Change-Id: I3b6e429b106aa22e58df25e0801b60af0fbd6d70
Signed-off-by: William A. Kennington III <wak@google.com>
diff --git a/include-fd/meson.build b/include-fd/meson.build
index 1e85293..3e36cb8 100644
--- a/include-fd/meson.build
+++ b/include-fd/meson.build
@@ -1,6 +1,7 @@
 stdplus_headers += include_directories('.')
 
 install_headers(
+  'stdplus/fd/atomic.hpp',
   'stdplus/fd/create.hpp',
   'stdplus/fd/dupable.hpp',
   'stdplus/fd/fmt.hpp',