introspection: remove the subdir from the header path

It is already included in the pkg-config file.

Signed-off-by: Corentin Noël <corentin.noel@collabora.com>

src/meson.build

@@ -171,7 +171,7 @@ if get_option('introspection')
     symbol_prefix: 'gxr',
     export_packages: api_path,
     includes: [ 'cairo-1.0', 'GdkPixbuf-2.0', 'Gdk-3.0', 'Graphene-1.0', 'Gulkan-'+api_version],
-    header: 'gxr/gxr.h',
+    header: 'gxr.h',
     install: true,
   )
 endif