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/src/meson.build b/src/meson.build
index f937dc7..91fecb5 100644
--- a/src/meson.build
+++ b/src/meson.build
@@ -24,6 +24,7 @@
 
 if has_fd
   stdplus_srcs += [
+    'fd/atomic.cpp',
     'fd/create.cpp',
     'fd/dupable.cpp',
     'fd/fmt.cpp',