build: Hide private structs from generated GIR files

Otherwise the fields in `MctAppFilter` become visible to introspected
languages, which was not the intention.

Spotted in
https://gitlab.freedesktop.org/pwithnall/malcontent/-/issues/65.

Signed-off-by: Philip Withnall <pwithnall@gnome.org>

libmalcontent/app-filter-private.h

@@ -48,6 +48,7 @@ typedef enum
 
 struct _MctAppFilter
 {
+  /*< private >*/
   gint ref_count;
 
   uid_t user_id;

libmalcontent/meson.build

@@ -66,7 +66,7 @@ pkgconfig.generate(libmalcontent,
 )
 
 libmalcontent_gir = gnome.generate_gir(libmalcontent,
-  sources: libmalcontent_sources + libmalcontent_headers + libmalcontent_private_headers + enums,
+  sources: libmalcontent_sources + libmalcontent_headers + enums,
   nsversion: libmalcontent_api_version,
   namespace: 'Malcontent',
   symbol_prefix: 'mct_',

libmalcontent/session-limits-private.h

@@ -50,6 +50,7 @@ typedef enum
 
 struct _MctSessionLimits
 {
+  /*< private >*/
   gint ref_count;
 
   uid_t user_id;