meson: install missing/generated headers

one headers (args/root.hh) was simply missing, and the generated headers
were not installed. not all of them *should* be installed either, only a
select few (and sadly this needs a custom target for each one, it seems)

Change-Id: I37b25517895d0e5e521abc1202fa65624de57ed1

src/libexpr/flake/meson.build

@@ -0,0 +1,8 @@
+libexpr_generated_headers += custom_target(
+  command : [ 'bash', '-c', 'echo \'R"__NIX_STR(\' | cat - @INPUT@ && echo \')__NIX_STR"\'' ],
+  input : 'call-flake.nix',
+  output : '@PLAINNAME@.gen.hh',
+  capture : true,
+  install : true,
+  install_dir : includedir / 'nix/flake',
+)