gienuminfo: Split out GIFlagsInfo as a derived type

Flag enums are already treated as a special kind of enum within
`GIInfoType`, so it would be tidier to give it its own `GType` too, with
a subtype relation to `GI_TYPE_ENUM_INFO`.

This will simplify implementing `GI_IS_ENUM_INFO` in a following commit
too.

Signed-off-by: Philip Withnall <pwithnall@gnome.org>
Helps: #3216

girepository/meson.build

@@ -49,6 +49,7 @@ girepo_headers = files(
   'giconstantinfo.h',
   'gienuminfo.h',
   'gifieldinfo.h',
+  'giflagsinfo.h',
   'gifunctioninfo.h',
   'giinterfaceinfo.h',
   'giobjectinfo.h',
@@ -149,6 +150,7 @@ girepo_sources = files(
   'giconstantinfo.c',
   'gienuminfo.c',
   'gifieldinfo.c',
+  'giflagsinfo.c',
   'gifunctioninfo.c',
   'ginvoke.c',
   'giinterfaceinfo.c',