include: Fix missing header install

Change-Id: Iaf4646f6c50616ccf9daa8288a06f450bb0003ef
Signed-off-by: William A. Kennington III <wak@google.com>
diff --git a/include/meson.build b/include/meson.build
index 67ad4f7..53fd254 100644
--- a/include/meson.build
+++ b/include/meson.build
@@ -4,6 +4,7 @@
   'stdplus/cancel.hpp',
   'stdplus/exception.hpp',
   'stdplus/flags.hpp',
+  'stdplus/pinned.hpp',
   'stdplus/raw.hpp',
   'stdplus/signal.hpp',
   'stdplus/zstring.hpp',