build: add missing source dirs to meson doc build

https://gitlab.freedesktop.org/NetworkManager/NetworkManager/-/merge_requests/1717

docs/libnm/meson.build

@@ -43,6 +43,8 @@ gnome.gtkdoc(
   src_dir: [
     libnm_core_public_inc,
     libnm_client_public_inc,
+    libnm_core_impl_inc,
+    libnm_client_impl_inc
   ],
   dependencies: libnm_dep,
   scan_args: scan_args,

src/libnm-client-impl/meson.build

@@ -1,5 +1,7 @@
 # SPDX-License-Identifier: LGPL-2.1-or-later
 
+libnm_client_impl_inc = include_directories('.')
+
 libnm_client_impl_sources = files(
   'nm-access-point.c',
   'nm-active-connection.c',